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.
- VCHam uses Mass-Frequency Scaled Normal Mode coordinates that are described
here
- Energies in the info file produced by vctrans are in eV,
and Cartesian coordinates in Bohr. A description of the transformations made is
here