MOLPRO

38 QUASI-DIABATIZATION 274
This calculation produces the following table:
Non-adiabatic couplings for LiF
R NACME1P NACME1M NACMEAV NACME2
10.0 -0.22828936 -0.22328949 -0.22578942 -0.22578942
10.5 -0.51777034 -0.50728914 -0.51252974 -0.51252974
11.0 0.76672943 0.76125391 0.76399167 0.76399167
11.5 0.42565202 0.42750263 0.42657733 0.42657733
12.0 0.19199878 0.19246799 0.19223338 0.19223338
Note that the sign changes because of a phase change of one of the wavefunctions. In order to
keep track of the sign, one has to inspect both the orbitals and the ci-vectors.
38 QUASI-DIABATIZATION
The DDR procedure can also be used to generate quasi-diabatic states and energies for MRCI
wavefucntions (CASSCF case can be treated as special case using the NOEXC directive in the
MRCI). The quasi-diabatic states have the propery that they change as little as possible relative
to a reference geometry; with other words, the overlap between the states at the current geometry
with those at a reference geometry is maximized by performing a unitary transformation among
the given states. Preferably, the adiabatic and diabatic states should be identical at the reference
geometry, e.g., due to symmetry. For instance, in the examples given below for the 1 B 1 and
1 A 2 states of H 2 S, C 2v geomtries are used as reference, and at these geometries the states are
unmixed due to their different symmetry. At the displaced geometries the molecular symmetry
is reduced to C S . Both states now belong to the 1 A ′′ irreducible representation and are strongly
mixed. For a description and application of the procedure described below, see D. Simah, B.
Hartke, and H.-J. Werner, J. Chem. Phys. 111, 4523 (1999).
This diabatization can be done automatically and requires two steps: first, the active orbitals
of a CASSCF calculation are rotated to maximize the overlap with the orbitals at the reference
geometry. This is achieved using the DIAB procedure described in section 19.5.8. Secondly, the
DDR procedure can be used to find the transformation among the CI vectors.
The following input is required:
DDR
ORBITAL,orb1, orb2
DENSITY,trdm1,trdm2
calls the DDR procedure.
orb1 and orb2 are the (diabatic) orbitals at the current and reference
geometry, respectively.
trdm1 are the transition densities computed at the current geometry,
trdm2 are transition densities computed using the wavefunctions of
the current (bra) and reference (ket) geometries.
MIXING,state1, state2, . . . The given states are included in the diabatization.
ENERGY,e1, e2, . . . Adiabatic energies of the states. If this input card is present, the
Hamiltonian in the basis of the diabatic states is computed and printed.
Alternatively, the energies can be passed to DDR using the Molpro
variable EADIA.