along direction . is the transition density between the two electronic states.

Evaluation of electronic wave functions for both electronic states are required at N displacement geometries for first order accuracy and 2*N displacements to achieve second order accuracy, where N is the number of nuclear degrees of freedom. This can be extremely computationally demanding for large molecules.

As with other numerical differentiation methods, the evaluation of nonadiabatic coupling vector with this method is numerically

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