MOLPRO

50 THE VCI PROGRAM (VCI) 365
50 THE VCI PROGRAM (VCI)
VCI,options
VCI calculations account for vibration correlation effects and are based on potential energy surface
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).
50.1 Options
The following options are available:
TYPE=value
VERSION=value
CITYPE=value
VCI solutions can be obtained using a potential in grid representation,
i.e. TYPE=GRID, or in a polynomial representation, TYPE=POLY. In
the latter case the POLY program needs to be called prior to the VSCF
and VCI programs in order to transform the potential.
Both, the grid-based and the polynomial-based versions of the VCI
programs offer 4 different kinds of VCI implementations: VERSION=1
is the fastest program. It works state-specific and configuration selective
and makes use of a simultaneous exclusion and internal contraction
scheme (see the reference given above). VERSION=2 is
identical with VERSION=1 but switches off the internal contractions.
VERSION=3 is the most accurate configuration selective VCI program
and does neither use the internal contraction scheme nor the simultaneous
exclusion. VERSION=4 is a conventional VCI program
without any of the aforementioned approximations. It is thus computationally
extremely demanding.
CTYPE defines the maximum number of simultaneous excitations, i.e.
Singles, Doubles, Triples, ... and thus determines the kind of calculations,
i.e. VCISD, VCISDT, ... The default is CITYPE=4 (VCIS-
DTQ), which appears to be a fair compromise between accuracy and
computational speed. The maximum excitation level is currently limited
to CITYPE=6.
LEVEX=value LEVEX determines the level of excitation within one mode, i.e. 0 → 1,
0 → 2, 0 → 3, ... The default is LEVEX=4, which was found to be
sufficient for many applications.
CIMAX=value
CIMAX is the maximum excitation level corresponding to CITYPE
and LEVEX. In principle, a triple configuration (4,4,4) would contribute
to the VCI space. However, CIMAX=7 restricts this to (4,3,0),
(3,3,1), (3,2,2),.... The default is CIMAX=8, which needs to be extended
for certain applications.