55.1 The VCI program (VCI)

VCI,options

VCI calculations account for vibration correlation effects and use potential energy surfaces as generated from the SURF program and a basis of VSCF modals. All VCI calculations will be performed state-specific, i.e. for each vibrational mode an individual VCI calculation will be performed. As VCI calculations may require substantial computer resources, these calculations can be rather expensive. Currently, two different VCI programs (configuration selective and conventional) are available (see below). Moreover, VCI calculations can be performed using the grid-based version of the program or within a polynomial representation. The latter is significantly faster and is thus recommended. The different versions of the configuration selection VCI program and the underlying configuration selection scheme are described in detail in:


M. Neff, G. Rauhut, Toward large scale vibrational configuration interaction calculations, J. Chem. Phys. 131, 124129 (2009).
M. Neff, T. Hrenar, D. Oschetzki, G. Rauhut, Convergence of vibrational angular momentum terms within the Watson Hamiltonian, J. Chem. Phys. 134, 064105 (2011).


The anharmonic frequencies and intensities calculated by the VCI program can be used to plot an IR spectrum, using the PUT command (see subsection 10.3) with the style IRSPEC.