Program announcement
"MULTIMODE"; a code to obtain accurate rovibrational energy levels for many-mode molecules by:
S. Carter, 39, Grove Hill, Caversham, Reading RG4 8PS, UK
J.M. Bowman, Department of Chemistry, Emory University, Atlanta GA 30322, USA
and contributions from N.C. Handy, Department of Chemistry, Lensfield Road,
Cambridge CB2 1EW, UK
"MULTIMODE" is a near-variational procedure for the calculation of
rovibrational energies of polatomic molecules, adsorbates and clusters.
The method is based on the Watson normal coordinate hamiltonian for
non-linear molecules, and proceeds via vibrational SCF and rovibrational
CI steps. The complete hamiltonian is used at all times, but approximations
are made in order to integrate the potential and coriolis terms. These terms
are included as N-mode expansions in the normal coordinates, but are
truncated at the 4-mode terms. It has been demonstrated that higher terms
are insignificant compared with the inaccuracies in the potential itself.
The method has been thoroughly tested for a variety of triatomic and
tetraatomic molecules and, in particular, against exact variational results
for H2CS (6 modes, ref. 1) and more recently it has been applied to
CH4 (9 modes, ref. 2). In both cases, excellent convergence
has been achieved for all of the low-lying vibrational levels.
"MULTIMODE" can be used in less accurate calculations, for example at the SCF
level alone, or in SCF-CI calculations, where specified numbers of vibrational
(non-orthogonal) SCF functions are used in a vibrational CI basis. These
methods also allow for an adiabatic rotation scheme, which gives reasonable
values for rotational constants, in particular.
"MULTIMODE" is currently being extended to include the Watson
hamiltonian for linear molecules, and also to include the Miller-Adams-Handy
Reaction Path hamiltonian in order to describe a single large-amplitude
coordinate, for example the torsion in H2O2. This will widen the scope of
molecules that can be handled.
information: www.emory.edu/CHEMISTRY/faculty/bowman/multimode
enquiries: [log in to unmask]
References
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1. S. Carter, J.M. Bowman and N.C.Handy; Theor. Chem. Accts., 100, 191 (1998)
2. S. Carter and J.M. Bowman; J. Phys. Chem. 104, 2443 (2000)
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