The VCMIN program can be used to perform a geometry optimisation on the model adiabatic surfaces of a VCHAM potential.

For an input file named 'file.inp', an output file named 'file.out' is created that contains the optimised geometry in both Cartesian and normal mode coordinates. Additionally, a file named 'file.xyz' is created that contains the optimised geometry in xyz format for viewing.

To run the program type
vcmin input
or
vcmin input.inp
where input (or input.inp) denotes the input file. The input format uses, like all the Quantics package input files, keywords that are for the most part free format and case insensitive. See Quantics input file structure

for further information on the general use of keywords, noting that there are no sections in the VCHAM input files. The input file ends with the keyword end-input


Input Keywords

The geometry optimisation is performed using a simple simplex optimiser. By default normal modes are used, but the use of polyspherical coordinates may be achieved using the keywords described here here.

Input Syntax
Keyword Description
geometry_define
end-geometry_define
The starting geometry (in xyz format) is given between these two keywords
file0 = S S is the file containing the normal modes and reference geometry of the vibronic coupling Hamiltonian
vcham_file = S S is the .vcham file created by VCHFIT
nstates = N N is the number of states
sopt = N N is the number of the state whose adiabatic potential is to be used in the optimisation
iter = N N is the number of iterations to be used in the optimisation


Examples

To illustrate the use of the VCMIN program, we consider the optimisation on the adiabatic potential surfaces provided by a vibronic coupling Hamiltonian parameterised to describe the first eight states of aniline. The input file used is named vcmin_aniline.inp. The .vcham file used is named all_fit.vcham. The normal modes used are contained in the file aniline_freq.log. The starting geomerty in the optimisation is specified between the lines geometry_define and end-geometry_define, and corresponds to the reference geometry used in the fitting of the model potential, which is the transition state for NH2 inversion in the ground state. The line sopt = 1 selects the potential surface of the ground state to perform the optimisation on. Running

vcmin vcmin_aniline.inp

results in the production of the output file vcmin_aniline.out, which contains the geometries of the starting and optimised geometries in both Cartesian and normal mode coordinates. Additionally the xyz file vcmin_aniline.xyz is produced, which contains the Cartesian coordinates of the optimised geometry for viewing purposes.