MOLPRO

36 DIABATIC ORBITALS 270
This procedure works as follows: first, the orbitals are determined at the reference geometry.
Then, the calculations are performed at displaced geometries, and the ”diabatic” active orbitals,
which have maximum overlap with the active orbitals at the reference geometry, are obtained
by adding a DIAB directive to the input:
Old form (Molpro96, obsolete):
DIAB,orbref, orbsav, orb1,orb2,pri
New form:
DIAB,orbref [,TYPE=orbtype] [,STATE=state] [,SPIN=spin] [,MS2=ms2] [,SAVE=orbsav]
[,ORB1=orb1, ORB2=orb2] [,PRINT=pri]
Here orbref is the record holding the orbitals of the reference geometry, and orbsav is the record
on which the new orbitals are stored. If orbsav is not given (recommended!) the new orbitals
are stored in the default dump record (2140.2) or the one given on the ORBITAL directive (see
section 19.5.3). In contrast to earlier versions of MOLPRO it is possible that orbref and orbsav
are the same. The specifications TYPE, STATE, SPIN can be used to select specific sets of
reference orbitals, as described in section 4.11. orb1, orb2 is a pair of orbitals for which the
overlap is to be maximized. These orbitals are specified in the form number.sym, e.g. 3.1 means
the third orbital in symmetry 1. If orb1, orb2 are not given, the overlap of all active orbitals is
maximized. pri is a print parameter. If this is set to 1, the transformation angles for each orbital
are printed for each jacobi iteration.
Using the defaults described above, the following input is sufficient in most cases:
DIAB,orbref
Using Molpro98 is is not necessary any more to give any GEOM and DISPL cards. The
displacements and overlap matrices are computed automatically (the geometries are stored in
the dump records, along with the orbitals).
The diabatic orbitals have the property that the sum of orbital and overlap contributions in the
non-adiabatic coupling matrix elements become approximately zero, such that the adiabatic
mixing occurs only through changes of the CI coefficients. This allows to determine the mixing
angle directly from the CI coefficients, either in a simple way as described for instance
in J. Chem. Phys. 89, 3139 (1988), or in a more advanced manner as described by Pacher,
Cederbaum, and Köppel in J. Chem. Phys. 89, 7367 (1988).
Below we present an example for the first two excited states of H 2 S, which have B 1 and A 2
symmetry in C 2v , and A ′′ symmetry in C S . We first perform a reference calculation in C 2v symmetry,
and then determine the diabatic orbitals for displaced geometries in C S symmetry. Each
subsequent calculation uses the previous orbitals as reference. One could also use the orbitals
of the C 2v calculation as reference for all other calculations. In this case one would have to take
out the second-last input card, which sets reforb=2141.2.