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| {{hatnote|See also [[adiabatic process]], a concept in [[thermodynamics]]}}
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| {{multiple issues |confusing=January 2011 |tone=April 2013}}
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| In [[quantum chemistry]], the [[potential energy surface]]s are obtained within the [[adiabatic process (quantum mechanics)|adiabatic]] or [[Born–Oppenheimer approximation]]. This corresponds to a representation of the molecular [[wave function]] where the variables corresponding to the [[molecular geometry]] and the electronic [[degrees of freedom (physics and chemistry)|degrees of freedom]] are [[separation of variable|separated]]. The [[vibronic coupling|non separable terms]] are due to the nuclear kinetic energy terms in the [[molecular Hamiltonian]] and are said to couple the [[potential energy surface]]s. In the neighbourhood of an [[avoided crossing]] or [[conical intersection]], these terms cannot be neglected. One therefore usually performs one [[unitary transformation]] from the [[adiabatic process (quantum mechanics)|adiabatic]] representation to the so-called '''diabatic representation''' in which the nuclear kinetic energy operator is [[diagonal matrix|diagonal]]. In this representation, the coupling is due to the [[ic molecular Hamiltonian|electronic energy]] and is a scalar quantity which is significantly easier to estimate numerically.
| |
| | |
| In the diabatic representation, the potential energy surfaces are smoother, so that low order [[Taylor series]] expansions of the surface capture much of the complexity of the original system. However strictly diabatic states do not exist in the general case. Hence, diabatic potentials generated from transforming multiple electronic energy surfaces together are generally not exact. These can be called '''pseudo-diabatic potentials''', but generally the term is not used unless it is necessary to highlight this subtlety. Hence, pseudo-diabatic potentials are synonymous with diabatic potentials.
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| | |
| ==Applicability==
| |
| The motivation to calculate diabatic potentials often occurs when the [[Born–Oppenheimer approximation]] does not hold, or is not justified for the molecular system under study. For these systems, it is necessary to go ''beyond'' the Born–Oppenheimer approximation. This is often the terminology used to refer to the study of [[nonadiabatic system]]s.
| |
| | |
| A well known approach involves recasting the molecular Schrödinger equation into a set of coupled eigenvalue equations. This is achieved by expansion of the exact wave function in terms of products of electronic and nuclear wave functions (adiabatic states) followed by integration over the electronic coordinates. The coupled operator equations thus obtained depend on nuclear coordinates only. Off-diagonal elements in these equations are nuclear kinetic energy terms. A diabatic transformation of the adiabatic states replaces these off-diagonal kinetic energy terms by potential energy terms. Sometimes, this is called the "adiabatic-to-diabatic transformation", abbreviated '''ADT'''.
| |
| <!--
| |
| We first show why this breakdown occurs and write <math>\mathbf{R}</math> for the collection of nuclear coordinates and <math>\mathbf{r}</math> for the electron coordinates of a [[molecule]] or cluster of molecules.
| |
| Indicating electronic and nuclear interactions by subscripts e and n, respectively, the [[Schrödinger equation|time-independent Schrödinger equation]] takes the form
| |
| | |
| :<math> H\Psi(\mathbf{R},\mathbf{r})\equiv
| |
| ( T_\mathrm{n} +T_\mathrm{e} + V_\mathrm{nn}+ V_\mathrm{ne} +V_\mathrm{ee}) \Psi(\mathbf{R},\mathbf{r})
| |
| = E \Psi(\mathbf{R},\mathbf{r}), </math>
| |
| where the kinetic energy terms <math>T_\mathrm{n}</math> and <math>T_\mathrm{e}</math> have the [[molecular Hamiltonian|usual form]]. In particular,
| |
| :<math> T_\mathrm{n} = \sum_{\alpha} \frac{P_\mathrm{n}^{\alpha} P_\mathrm{n}^{\alpha}}{2M_\alpha} </math> | |
| with the nuclear momentum
| |
| :<math> P_\mathrm{n}^{\alpha} = -i \partial /\partial R_\alpha. </math>
| |
| | |
| The wave function is expanded in ''M'' electronic eigenfunctions <math>\chi_k (\mathbf{r}; \mathbf{R})</math> of <math>H_\mathrm{e} \equiv H - T_\mathrm{n} \,\; </math>
| |
| :<math> \Psi(\mathbf{R}, \mathbf{r}) = \sum_{k=1}^M \chi_k(\mathbf{r};\mathbf{R}) \phi_k(\mathbf{R}) </math>
| |
| with
| |
| :<math> \langle\,\chi_{k'}(\mathbf{r};\mathbf{R})\,|\, \chi_k(\mathbf{r};\mathbf{R})\rangle_{(\mathbf{r})} = \delta_{k' k} </math>
| |
| and where the subscript <math>(\mathbf{r})</math> indicates that the integration is over electronic coordinates only. By definition, the matrix with general element
| |
| :<math> \big(\mathbb{H}_\mathrm{e}(\mathbf{R})\big)_{k'k} \equiv \langle \chi_{k'}(\mathbf{r};\mathbf{R})
| |
| | H_\mathrm{e} |
| |
| \chi_k(\mathbf{r};\mathbf{R})\rangle_{(\mathbf{r})} = \delta_{k'k} E_k(\mathbf{R})
| |
| </math>
| |
| is diagonal. We assume that this matrix is real, i.e., that there are no magnetic or spin interactions. After multiplication by <math>\chi_{k'}(\mathbf{r};\mathbf{R})</math> and integration over the electronic coordinates <math>\mathbf{r}</math> the Schrödinger equation is turned into a set of ''M'' coupled eigenvalue equations depending on nuclear coordinates only
| |
| | |
| :<math> \left[ \mathbb{H}_\mathrm{n}(\mathbf{R}) + \mathbb{H}_\mathrm{e}(\mathbf{R}) \right]
| |
| \boldsymbol{\phi}(\mathbf{R}) = E \boldsymbol{\phi}(\mathbf{R}).
| |
| </math>
| |
| | |
| The column vector <math>\boldsymbol{\phi}(\mathbf{R})</math> has elements <math>\phi_k(\mathbf{R}),\; k=1,\ldots,M</math>. The matrix <math>\mathbb{H}_\mathrm{e}(\mathbf{R})</math> is diagonal and the nuclear Hamilton matrix is non-diagonal with the following off-diagonal terms,
| |
| :<math>
| |
| \big(\mathbb{H}_\mathrm{n}(\mathbf{R})\big)_{k'k}
| |
| = \langle\chi_{k'}(\mathbf{r};\mathbf{R}) |
| |
| T_\mathrm{n}|\chi_k(\mathbf{r};\mathbf{R})\rangle_{(\mathbf{r})}.
| |
| </math>
| |
| | |
| Clearly, the off-diagonal coupling is by nuclear kinetic energy terms. Suppressing the coordinates in the notation, we can write, by applying the [[Leibniz rule (generalized product rule)|Leibniz rule]] for differentiation, the matrix elements of <math>T_{\textrm{n}}</math> as
| |
| :<math> \langle\chi_{k'}|T_\mathrm{n}|\chi_k \rangle_{(\mathbf{r})} = \delta_{k'k} T_{\textrm{n}}
| |
| + \sum_{\alpha}\frac{1}{M_\alpha} \langle\chi_{k'}|\big(P_\mathrm{n}^{\alpha}\chi_k\big)\rangle_{(\mathbf{r})} P_\mathrm{n}^{\alpha} + \langle\chi_{k'}|\big(T_\mathrm{n}\chi_k\big)\rangle_{(\mathbf{r})}
| |
| </math>
| |
| | |
| The diagonal (<math>k'=k</math>) matrix elements <math>\langle\chi_{k}|\big(P_\mathrm{n}^{\alpha}\chi_k\big)\rangle_{(\mathbf{r})}</math> of the operator <math>P_\mathrm{n}^{\alpha}</math> vanish, because this operator is Hermitian and purely imaginary. The off-diagonal matrix elements satisfy
| |
| | |
| :<math>
| |
| \langle\chi_{k'}|\big(P_\mathrm{n}^{\alpha}\chi_k\big)\rangle_{(\mathbf{r})}
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| =
| |
| \frac{\langle\chi_{k'}
| |
| |\big[P_\mathrm{n}^{\alpha}, H_\mathrm{e}\big] |
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| \chi_k\rangle_{(\mathbf{r})}} {E_{k}(\mathbf{R})- E_{k'}(\mathbf{R})}.
| |
| </math>
| |
| | |
| We see that whenever two surfaces come close, <math>{E_{k}(\mathbf{R})\approx E_{k'}(\mathbf{R})}</math>, the nuclear momentum coupling term is no longer negligible. Conversely, if all surfaces are well separated, all off-diagonal terms can be neglected and hence the whole matrix of <math>P^{\alpha}_\mathrm{n}</math> is effectively zero. The third term on the right hand side of the one but last equation can be written as the matrix of <math>P^{\alpha}_\mathrm{n}</math> squared and, accordingly, is then negligible also. Only the first (diagonal) kinetic energy term in this equation survives and a diagonal, uncoupled, set of nuclear motion equations results. These are the normal nuclear motion equations that appear in the second-step of the Born-Oppenheimer approximation.
| |
| --> | |
| | |
| ==Diabatic transformation of two electronic surfaces==
| |
| In order to introduce the diabatic transformation we assume now, for the sake of argument, that only two Potential Energy Surfaces (PES), 1 and 2, approach each other and that all other surfaces are well separated; the argument can be generalized to more surfaces. Let the collection of electronic coordinates be indicated by <math>\mathbf{r}</math>, while <math>\mathbf{R}</math> indicates dependence on nuclear coordinates. Thus, we assume <math>E_1(\mathbf{R}) \approx
| |
| E_2(\mathbf{R})</math> with corresponding orthonormal electronic eigenstates
| |
| <math>\chi_1(\mathbf{r};\mathbf{R})\,</math> and <math>\chi_2(\mathbf{r};\mathbf{R})\,</math>.
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| In the absence of magnetic interactions these electronic states, which depend parametrically on the nuclear coordinates, may be taken to be real-valued functions.
| |
| | |
| The nuclear kinetic energy is a sum over nuclei ''A'' with mass ''M''<sub>A</sub>,
| |
| :<math> T_\mathrm{n} = \sum_{A} \sum_{\alpha=x,y,z} \frac{P_{A\alpha} P_{A\alpha}}{2M_A}
| |
| \quad\mathrm{with}\quad
| |
| P_{A\alpha} = -i \nabla_{A\alpha} \equiv -i \frac{\partial\quad}{\partial R_{A\alpha}}. </math>
| |
| ([[Atomic units]] are used here).
| |
| By applying the [[Leibniz rule (generalized product rule)|Leibniz rule]] for differentiation, the matrix elements of <math>T_{\textrm{n}}</math> are (where we suppress coordinates for clarity reasons):
| |
| :<math>
| |
| \mathrm{T_n}(\mathbf{R})_{k'k}
| |
| \equiv \langle \chi_{k'} | T_n | \chi_k\rangle_{(\mathbf{r})}
| |
| = \delta_{k'k} T_{\textrm{n}}
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| + \sum_{A,\alpha}\frac{1}{M_A} \langle\chi_{k'}|\big(P_{A\alpha}\chi_k\big)\rangle_{(\mathbf{r})} P_{A\alpha} + \langle\chi_{k'}|\big(T_\mathrm{n}\chi_k\big)\rangle_{(\mathbf{r})}.
| |
| </math>
| |
| The subscript <math>{(\mathbf{r})}</math> indicates that the integration inside the braket is | |
| over electronic coordinates only.
| |
| Let us further assume
| |
| that all off-diagonal matrix elements
| |
| <math>\mathrm{T_n}(\mathbf{R})_{kp} = \mathrm{T_n}(\mathbf{R})_{pk}
| |
| </math> may be neglected except for ''k = 1'' and
| |
| ''p = 2''. Upon making the expansion
| |
| :<math>
| |
| \Psi(\mathbf{r},\mathbf{R}) = \chi_1(\mathbf{r};\mathbf{R})\Phi_1(\mathbf{R})+
| |
| \chi_2(\mathbf{r};\mathbf{R})\Phi_2(\mathbf{R}),
| |
| </math>
| |
| the coupled Schrödinger equations for the nuclear part take the form (see the article [[Born–Oppenheimer approximation]]) | |
| | |
| <math>
| |
| \begin{pmatrix}
| |
| E_1(\mathbf{R})+ \mathrm{T_n}(\mathbf{R})_{11}&\mathrm{T_n}(\mathbf{R})_{12}\\
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| \mathrm{T_n}(\mathbf{R})_{21}&E_2(\mathbf{R})+\mathrm{T_n}(\mathbf{R})_{22}\\
| |
| \end{pmatrix}
| |
| \boldsymbol{\Phi}(\mathbf{R})
| |
| = E \,\boldsymbol{\Phi}(\mathbf{R})
| |
| \quad \mathrm{with}\quad
| |
| \boldsymbol{\Phi}(\mathbf{R})\equiv
| |
| \begin{pmatrix}
| |
| \Phi_1(\mathbf{R}) \\
| |
| \Phi_2(\mathbf{R}) \\
| |
| \end{pmatrix} .
| |
| </math>
| |
| | |
| In order to remove the problematic off-diagonal kinetic energy terms, we
| |
| define two new orthonormal states by a '''diabatic transformation''' of the '''adiabatic states''' <math>\chi_{1}\,</math> and <math>\chi_{2}\,</math>
| |
| :<math>
| |
| \begin{pmatrix}
| |
| \varphi_1(\mathbf{r};\mathbf{R}) \\
| |
| \varphi_2(\mathbf{r};\mathbf{R}) \\
| |
| \end{pmatrix}
| |
| =
| |
| \begin{pmatrix}
| |
| \cos\gamma(\mathbf{R}) & \sin\gamma(\mathbf{R}) \\
| |
| -\sin\gamma(\mathbf{R}) & \cos\gamma(\mathbf{R}) \\
| |
| \end{pmatrix}
| |
| \begin{pmatrix}
| |
| \chi_1(\mathbf{r};\mathbf{R}) \\
| |
| \chi_2(\mathbf{r};\mathbf{R}) \\
| |
| \end{pmatrix}
| |
| </math>
| |
| where <math>\gamma(\mathbf{R})</math> is the '''diabatic angle'''. Transformation of the matrix of nuclear momentum <math>\langle\chi_{k'}|\big(P_{A\alpha}\chi_k\big)\rangle_{(\mathbf{r})}</math> for <math>k', k =1,2</math> gives for ''diagonal'' matrix elements
| |
| :<math> \langle{\varphi_k} |\big( P_{A\alpha} \varphi_k\big) \rangle_{(\mathbf{r})} = 0 \quad\textrm{for}\quad k=1, \, 2.
| |
| </math>
| |
| These elements are zero because <math>\varphi_k</math> is real
| |
| and <math>P_{A\alpha}\,</math> is Hermitian and pure-imaginary.
| |
| The off-diagonal elements of the momentum operator satisfy, | |
| :<math> \langle{\varphi_2} |\big( P_{A\alpha}\varphi_1\big) \rangle_{(\mathbf{r})} = \big(P_{A\alpha}\gamma(\mathbf{R}) \big) + \langle\chi_2| \big(P_{A\alpha} \chi_1\big)\rangle_{(\mathbf{r})}.
| |
| </math>
| |
| | |
| Assume that a diabatic angle <math>\gamma(\mathbf{R})</math> exists, such that to a good approximation
| |
| :<math> \big(P_{A\alpha}\gamma(\mathbf{R})\big)+ \langle\chi_2|\big(P_{A\alpha} \chi_1\big) \rangle_{(\mathbf{r})} = 0 </math>
| |
| | |
| i.e., <math>\varphi_1</math> and <math>\varphi_2</math> diagonalize the 2 x 2 matrix of the nuclear momentum. By the definition of
| |
| Smith<ref name="smith">{{cite journal |last=Smith |first=F.T. |title=Diabatic and Adiabatic Representations for Atomic Collision Problems |year = 1969 |journal = Physical Review |volume = 179 |issue=1 |pages=111–123|publisher=[[American Physical Society]] |doi=10.1103/PhysRev.179.111 |bibcode = 1969PhRv..179..111S }}</ref> <math>\varphi_1</math> and <math>\varphi_2</math> are '''diabatic states'''. (Smith was the first to define this concept; earlier the term ''diabatic'' was used somewhat loosely by Lichten <ref name="lichten">{{cite journal |last=Lichten |first=W. |title=Resonant Charge Exchange in Atomic Collisions |year = 1963 |journal = Physical Review |volume = 131 |issue=1 |pages=229–238 |publisher=[[American Physical Society]] |doi=10.1103/PhysRev.131.229 |bibcode = 1963PhRv..131..229L }}</ref>).
| |
| | |
| By a small change of notation these differential equations for <math>\gamma(\mathbf{R})</math> can be rewritten in the following more familiar form:
| |
| :<math>
| |
| F_{A\alpha}(\mathbf{R}) = - \nabla_{A\alpha} V(\mathbf{R})
| |
| \qquad\mathrm{with}\;\; V(\mathbf{R}) \equiv \gamma(\mathbf{R})\;\;\mathrm{and}\;\;F_{A\alpha}(\mathbf{R})\equiv
| |
| \langle\chi_2|\big(iP_{A\alpha} \chi_1\big) \rangle_{(\mathbf{r})} .
| |
| </math>
| |
| It is well known that the differential equations have a solution (i.e., the "potential" ''V'' exists) if and only if the vector field ("force")
| |
| <math>F_{A\alpha}(\mathbf{R})</math> is [[irrotational]],
| |
| :<math>
| |
| \nabla_{A\alpha} F_{B\beta}(\mathbf{R}) - \nabla_{B \beta} F_{A\alpha}(\mathbf{R}) = 0.
| |
| </math>
| |
| It can be shown that these conditions are rarely ever satisfied, so that a strictly diabatic
| |
| transformation rarely ever exists. It is common to use approximate functions <math>\gamma(\mathbf{R})</math> leading to ''pseudo diabatic states''.
| |
| | |
| Under the assumption that the momentum operators are represented exactly by 2 x 2 matrices, which is consistent with neglect of off-diagonal elements other than the (1,2) element and
| |
| the assumption of "strict" diabaticity,
| |
| it can be shown that
| |
| :<math>
| |
| \langle \varphi_{k'} | T_n | \varphi_k \rangle_{(\mathbf{r})} = \delta_{k'k} T_n.
| |
| </math>
| |
| | |
| On the basis of the diabatic states
| |
| the nuclear motion problem takes the following ''generalized Born–Oppenheimer'' form
| |
| <math>
| |
| \begin{pmatrix}
| |
| T_\mathrm{n}+
| |
| \frac{E_{1}(\mathbf{R})+E_{2}(\mathbf{R})}{2} & 0 \\
| |
| 0 & T_\mathrm{n} +
| |
| \frac{E_{1}(\mathbf{R})+E_{2}(\mathbf{R})}{2}
| |
| \end{pmatrix}
| |
| \tilde{\boldsymbol{\Phi}}(\mathbf{R})
| |
| +
| |
| \tfrac{E_{2}(\mathbf{R})-E_{1}(\mathbf{R})}{2}
| |
| \begin{pmatrix}
| |
| \cos2\gamma
| |
| & \sin2\gamma \\
| |
| \sin2\gamma &
| |
| -\cos2\gamma
| |
| \end{pmatrix}
| |
| \tilde{\boldsymbol{\Phi}}(\mathbf{R})
| |
| = E \tilde{\boldsymbol{\Phi}}(\mathbf{R}).
| |
| </math>
| |
| | |
| It is important to note that the off-diagonal elements depend on the diabatic angle and electronic energies only. The surfaces <math>E_{1}(\mathbf{R})</math> and <math>E_{2}(\mathbf{R})</math> are adiabatic PESs obtained from clamped nuclei electronic structure calculations and <math>T_\mathrm{n}\,</math> is the usual nuclear kinetic energy operator defined above.
| |
| Finding approximations for <math>\gamma(\mathbf{R})</math> is the remaining problem before a solution of the Schrödinger equations can be attempted. Much of the current research in quantum chemistry is devoted to this determination. Once <math>\gamma(\mathbf{R})</math> has been found and the coupled equations have been solved, the final vibronic wave function in the diabatic approximation is
| |
| :<math>
| |
| \Psi(\mathbf{r},\mathbf{R}) = \varphi_1(\mathbf{r};\mathbf{R})\tilde\Phi_1(\mathbf{R})+
| |
| \varphi_2(\mathbf{r};\mathbf{R})\tilde\Phi_2(\mathbf{R}).
| |
| </math>
| |
| <!--
| |
| with | |
| :<math>
| |
| \tilde{\boldsymbol{\Phi}}(\mathbf{R}) \equiv
| |
| \begin{pmatrix}
| |
| \tilde\Phi_1(\mathbf{R}) \\
| |
| \tilde\Phi_2(\mathbf{R}) \\
| |
| \end{pmatrix}
| |
| =
| |
| \begin{pmatrix}
| |
| \cos\gamma(\mathbf{R}) & \sin\gamma(\mathbf{R}) \\
| |
| -\sin\gamma(\mathbf{R}) & \cos\gamma(\mathbf{R}) \\
| |
| \end{pmatrix}
| |
| \begin{pmatrix}
| |
| \Phi_1(\mathbf{R}) \\
| |
| \Phi_2(\mathbf{R}) \\
| |
| \end{pmatrix}.
| |
| </math>
| |
| -->
| |
| | |
| ==Adiabatic-to-diabatic transformation==
| |
| Here, in contrast to previous treatments, the [[Non-abelian group|non-Abelian]] case is considered.
| |
| | |
| Felix Smith in his article<ref>F. T. Smith, Phys. Rev. 179, 111 (1969)</ref> considers the adiabatic-to-diabatic transformation (ADT) for a multi-state system but a single coordinate, <math>\mathrm{R_{A\alpha}}</math>. In Diabatic, the ADT is defined for a system of two coordinates <math>\mathrm{R_{A\alpha}}</math> and <math>\mathrm{R_{B\beta}}</math>, but it is restricted to two states. Such a system is defined as [[Abelian]] and the ADT matrix is expressed in terms of an angle, <math>\gamma</math> (see Comment below), known also as the ADT angle. In the present treatment a system is assumed that is made up of ''M'' (> 2) states defined for an ''N''-dimensional configuration space, where ''N'' = 2 or ''N'' > 2. Such a system is defined as non-Abelian.
| |
| To discuss the non-Abelian case the equation for the just mentioned ADT angle, <math>\gamma</math> (see Diabatic), is replaced by an equation for the MxM, ADT matrix, <math>\mathbf{A}</math>:<ref name="M. Baer, Chem 1975">M. Baer, Chem. Phys. Lett. 35, 112 (1975)</ref>
| |
| | |
| : <math>\nabla \mathbf{ A + FA = 0}</math>
| |
| | |
| where <math>\mathbf{F} </math> is the force-matrix operator, introduced in Diabatic, also known as the Non-Adiabatic Coupling Transformation (NACT) matrix:<ref>M. Born and K. Huang, Dynamical Theory of Crystal Lattices, 1954 (Oxford Univ. Press, New York), Chap. IV</ref>
| |
| | |
| : <math>\ \mathbf{F}_{jk} = \langle \chi_j\mid \nabla \chi_k\rangle;\qquad j,k=1,2,\ldots,M </math>
| |
| | |
| Here <math> \nabla</math> is the ''N''-dimensional (nuclear) grad-operator:
| |
| | |
| : <math>\nabla = \left\{ \frac{\partial\quad}{\partial q_{1}},\quad \frac{\partial\quad}{\partial q_{2}} , \ldots, \frac{\partial\quad}{\partial q_N} \right\}</math>
| |
| | |
| and <math>|\chi_k( \mathbf{r\mid q} ) \rangle;\ k= 1,M</math>,are the electronic adiabatic eigenfunctions which depend explicitly on the electronic coordinates <math>\mathbf{r} </math> and parameterically on the nuclear coordinates <math>\mathbf{q} </math>.
| |
| | |
| To derive the matrix <math>\mathbf{A} </math> one has to solve the above given first order differential equation along a specified contour <math>\Gamma </math>. This solution is then applied to form the diabatic potential matrix <math>\mathbf{W} </math>:
| |
| | |
| : <math>\mathbf{W} = \mathbf{A}^{*}\mathbf{uA}</math> | |
| | |
| where <math>\mathbf{u}_j</math> ; ''j'' = 1, ''M'' are the [[Born–Oppenheimer]] adiabatic potentials. In order for <math>\mathbf{W} </math> to be single-valued in configuration space, <math>\mathbf{A} </math> has to be [[analytic]] and in order for <math>\mathbf{A} </math> to be analytic (excluding the pathological points), the components of the vector matrix, <math>\mathbf{F} </math>, have to satisfy the following equation:<ref>M. Baer, Beyond Born Oppenheimer; Electronic non-Adiabatic coupling Terms and Conical Intersections, 2006 (Wiley & Sons Inc, Hoboken N.J.); Chap. 1</ref><ref>R. Englman and A. Yahalom, Adv. Chem. Phys. 124, 197. (2002)</ref>
| |
| | |
| : <math> G_{{q_i }{q_j}}= \frac{{\partial}\mathbf{F}_{q_i}}{\partial q_{j}} - \frac{{\partial}\mathbf{F}_{q_j}}{\partial q_i} - \left[ \mathbf{F}_{q_i} , \mathbf{F}_{q_j} \right] = 0. </math>
| |
| | |
| where <math>\mathbf{G} </math> is a [[tensor field]]. This equation is known as the non-Abelian form of the [[Curl (mathematics)|Curl]] Equation.
| |
| A solution of the ADT matrix <math>\mathbf{A}</math> along the contour <math>\Gamma</math> can be shown to be of the form:<ref>M. Baer, Molec. Phys. 40, 1011 (1980)</ref><ref>D.R. Yarkony, in: W. Domcke, D.R. Yarkony and H. Köppel, Eds., Conical Intersections: Electronic Structure, Dynamics and Spectroscopy, (Singapore: World Sci. 2004</ref><ref>I. Ryb and R. Baer, J. Chem. Phys. 121, 10370 (2004)</ref>
| |
| | |
| : <math>\mathbf{A}\left( \mathbf{q}|\Gamma \right) = \hat{P} \exp </math>
| |
| | |
| : <math> \left( - \int_\mathbf{q_0}^\mathbf{q}\mathbf{F} \left( \mathbf{q'}\mid \Gamma \right) \cdot d\mathbf{q'}\right)</math>
| |
| | |
| (see also [[Geometric phase]]). Here <math>\hat{P}</math> is an [[ordering operator]], the dot stands for a [[scalar product]] and <math>\mathbf{q}</math> and <math>\mathbf{q_0}</math> are two points on <math>\Gamma</math>.
| |
| | |
| A different type of solutions is based on quasi-Euler angles according to which any <math>\mathbf{A}</math>-matrix can be expressed as a product of [[Euler matrices.]]<ref>Z.H. Top and M. Baer, J. Chem. Phys. 66, 1363 (1977); M. Baer, S.H. Lin, A. Alijah, S. Adhikari and G.D. Billing, Phys. Rev. A 62, 032506 (2000)</ref> For instance in case of a tri-state system this matrix can be presented as a product of three such matrices, <math>\mathbf{Q}_{j}(\gamma_{ij})</math> (''i'' < ''j'' = 2, 3) where e.g. <math>\mathbf{Q}_{13} (\gamma_{13})</math> is of the form:
| |
| | |
| : <math>\mathbf{Q}_{13} = \begin{pmatrix} \cos \gamma_{13} & 0 & \sin\gamma_{13}\\0 & 1 & 0\\
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| -\sin\gamma_{13} & 0 & \cos\gamma_{13} \end{pmatrix}</math>
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| The product <math>\mathbf{A} = \mathbf{Q}_{kl} \mathbf{Q}_{mn} \mathbf{Q}_{pq}</math> which can be written in any order, is substituted in Eq. (1) to yield three first order differential equations for the three <math>{\gamma}_{ij}</math>-angles where two of these equations are coupled and the third stands on its own. Thus, assuming: <math>\mathbf{A} = \mathbf{Q}_{12} \mathbf{Q}_{23} \mathbf{Q}_{13}</math> the two coupled equations for <math>{\gamma}_{12}</math> and <math>{\gamma}_{23}</math> are:
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| : <math>\nabla \gamma_{12} = -F_{12} -
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| \tan{\gamma}_{23} (- F_{13} \cos\gamma_{12} + F_{23}\sin \gamma_{12})</math>
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| : <math>\nabla \gamma_{23} = - (F_{23} \cos\gamma_{12} + F_{13} \sin\gamma_{12})</math>
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| whereas the third equation (for <math>\gamma_{13}</math>) becomes an ordinary (line) integral:
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| : <math>\nabla \gamma_{13} =(\cos\gamma_{23})^{-1}(- F_{13}\cos\gamma_{12} + F_{23}\sin\gamma_{12})</math>
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| expressed solely in terms of <math>\gamma_{12}</math> and <math>\gamma_{23}</math>.
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| Similarly, in case of a four-state system <math>\mathbf{A}</math> is presented as a product of six 4 x 4 Euler matrices (for the six quasi-Euler angles) and the relevant six differential equations form one set of three coupled equations, whereas the other three become, as before, ordinary line integrals.<ref>B. Sarkar and S. Adhikari, J. Phys. Chem. A 112, 9868 (2008); A. Das and D. Mukhopadhyay, J. Phys. Chem. A 116, 1774 (2012)</ref>
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| ==A comment concerning the two-state (Abelian) case==
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| Since the treatment of the two-state case as presented in Diabatic raised numerous doubts we consider it here as a special case of the [[Non-Abelian]] case just discussed.
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| For this purpose we assume the 2 × 2 ADT matrix <math>\mathrm{A}</math> to be of the form:
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| : <math>\mathrm{A} = \begin{pmatrix}\cos\gamma & - \sin\gamma\\\sin\gamma & \cos\gamma \end{pmatrix}</math>
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| Substituting this matrix in the above given first order differential equation (for <math>\mathrm{A}</math>) we get, following a few algebraic rearrangements, that the angle <math>\gamma</math> fulfills the corresponding first order differential equation as well as the subsequent line integral:<ref name="M. Baer, Chem 1975"/><ref>T. Pacher, L.S. Cederbaum and H. Koppel, Adv. Chem. Phys. 84, 293 (1993)</ref><ref>D.R. Yarkony, J. Chem. Phys. 105, 10456 (1996)</ref><ref>M. Baer, Beyond Born–Oppenheimer: Electronic Non-Adiabatic Coupling Terms and Conical Intersections (Wiley & Sons,, Inc., Hoboken,, N.J., 2006), Chapter 3</ref><ref>R. Baer, Phys. Rev. Let. 104, 073001 (2001)</ref>
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| : <math>\nabla \mathbf{ \gamma + F_{12} = 0 } \cdot \Longrightarrow \cdot\gamma\left( \mathbf{q}\mid \Gamma \right) = -\int_\mathbf{q_0}^\mathbf{q}\mathbf{F}_{12} \left( \mathbf{q'}\mid \Gamma \right) \cdot d\mathbf{q'}</math> | |
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| where <math>\mathrm{F}_{12}</math> is the relevant [[NACT]] matrix element, the dot stands for a scalar product and <math>\Gamma</math> is a chosen contour in configuration space (usually a planar one) along which the integration is performed.
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| The line integral yields meaningful results if and only if the corresponding (previously derived) [[Curl (mathematics)|Curl]]-equation is zero for every point in the region of interest (ignoring the pathological points).
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| ==References==
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| <references/>
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| [[Category:Quantum chemistry]]
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| [[Category:Quantum mechanics]]
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