Vibrational Circular Dichroism calculations

The calculation of vibrational circular dichroism   is invoked by the keyword .VCD   in the ABACUS  input module. Thus a complete input file for the calculation of vibrational circular dichroism will look like:

**DALTON INPUT
.RUN PROPERTIES
**WAVE FUNCTIONS
.HF
**PROPERTIES
.VCD
*END OF INPUT

This will invoke the calculation of vibrational circular dichroism using London atomic orbitals  to ensure fast basis set convergence as well as gauge origin  independent results. By default the natural connection  is used in order to get numerically accurate results [34, 50].

We notice, however, that vibrational circular dichroism only arises in vibrationally chiral molecules. An easy way of introducing vibrational chirality into small molecular systems is by isotopic substitution. This is in DALTON controled in the *VIBANA   submodule, and the reader is refered to that section for an exemplification of how this is done.

There has been a couple of investigation of basis set requirement for the calculation of VCD given in Ref. [51, 52], and the reader is refered to these references when choosing basis set for the calculations of VCD.

In the current implementation, the .NOCMC   option is autmatically turned on in VCD calculations, that is, the coordinate system origin is always used as gauge origin .

We note that if a different force field is wanted in the calculation of the VCD paramaters, this can be obtained by reading in an alternative Hessian  matrix with the input

**DALTON INPUT
.RUN PROPERTIES
**WAVE FUNCTIONS
.HF
**PROPERTIES
.VCD
*VIBANA
.HESFIL
*END OF INPUT

If more close control of the different parts of the calculation of vibrational circular dichroism is wanted, we refer the reader to the sections describing the options available. The input sections that control the calculation of vibrational circular dichroism are:

*AAT  

Controls the final calculation of the different contributions to the Atomic Axial Tensors.

*GETSGY  

Controls the set up of both the magnetic and geometric right hand sides. (gradient terms)

*LINRES  

Controls the solution of the magnetic response equations.

*RELAX  

Controls the adding of solution and right hand side vectors into relaxation contributions.

*NUCREP  

Controls the calculation of the nuclear contribution to the geometric Hessian.

*TROINV  

Controls the use of translation and rotational invariance.

*ONEINT  

Controls the calculation of one electron contributions to the geometric Hessian.

*TWOEXP  

Controls the calculation of two-electron expectation values to the geometric Hessian.

*REORT  

Controls the calculation of reorthonormalization terms to the geometric Hessian.

*RESPON  

Controls the solution of the geometric response equations.

*GEOANA  

Describes what analysis of the molecular geometry is to be printed.

*VIBANA  

Set up the vibrational and rotational analysis of the molecule, for instance it's isotopic substitution.

*DIPCTL  

Controls the calculation of the Atomic Polar Tensors (dipole gradient).