Potential energy surfaces (PES) of molecular systems often exhibit a conical intersection. This conical intersection allows very fast transitions between the electronic states.
Due to the dependence of this mechanism to the molcular vibration, this process is denoted as vibronic coupling (VC).

VCHam is a program to derive from a given data set of (ab initio) energy points of two (or more) potential energy surfaces (PES) at given molecular geometries a realistiv model of the vibronically coupled Hamiltonian. We are using an expansion in normal modes of the VC Hamiltonian, comprising terms up to the bilinear and quadratic order.

VCHam is able to choose separately each model parameter. Hence, a treatment of systems, beginning from a simple linear model up to full quadratic model, is easily feasible. The computation of the parameters is performed by a least-square fit algorithm.

The VCHam output is suited for the use with the Heidelberg MCTDH (multiconfiguration time-dependent) program.

The VCHam algorithm can be used iteratively to improve gradually the VC Hamiltonian by including additional ab initio energy points of the PES.

It is useful to start with a linear model. When there is enough confidence in the resulting linear VC model parameters, the VC Hamiltonian can be expanded stepwise by including additional bilinear and quadratic parameters into the fitting algorithm.

The advantage of this method is the possibility to include energy points  at larger distances from the Franck-Condon zone. The inclusion of such energy points leads to a more reliable model of the VC Hamiltonian.


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