MOLPRO

36 DIABATIC ORBITALS 269
DKHP=3:
DKHP=4:
DKHP=5:
Square-root parametrization (SQR)
McWeeny parametrization (MCW)
Cayley parametrization (CAY)
Example:
SET,DKROLL=1 ! activate Douglas–Kroll–Hess one-electron integrals
SET,DKHO=8 ! DKH order = 8
SET,DKHP=4 ! choose McWeeny parametrization for unitary transformations
(Note: For DKHO ≥ 13 the values of some parameters in the file src/common/dkhparameters.inc
have to be suitably increased. Only recommended for experts who do exactly know what they
are doing!! For most cases DKHO=10 is sufficient.)
Up to fourth order (DKHO=4) the DKH Hamiltonian is independent of the chosen paramterization.
Higher-order DKH Hamiltonians depend slightly on the chosen paramterization of the
unitary transformations applied in order to decouple the Dirac Hamiltonian.
For details on the infinite-order DKH Hamiltonians see
M. Reiher, A. Wolf, JCP 121, 2037–2047 (2004),
M. Reiher, A. Wolf, JCP 121, 10945–10956 (2004).
For details on the different parametrizations of the unitary transformations see
A. Wolf, M. Reiher, B. A. Hess, JCP 117, 9215–9226 (2002).
35.2 Example for computing relativistic corrections
***,ar2
geometry={ar1;ar2,ar1,r}
r=2.5 ang
{hf;
expec,rel,darwin,massv}
e_nrel=energy
show,massv,darwin,erel
dkroll=1
hf;
e_dk=energy
show,massv,darwin,erel
show,e_dk-e_nrel
!geometry definition
!bond distance
!non-relativisitic scf calculation
!compute relativistic correction using Cowan-Griffin operator
!save non-relativistic energy in variable enrel
!show individual contribution and their sum
!use douglas-kroll one-electron integrals
!relativistic scf calculation
!save relativistic scf energy in variable e_dk.
!show mass-velocity and darwin contributions and their sum
!show relativistic correction using Douglas-Kroll
http://www.molpro.net/info/current/examples/ar2_rel.com
36 DIABATIC ORBITALS
In order to construct diabatic states, it is necessary to determine the mixing of the diabatic
states in the adiabatic wavefunctions. In principle, this mixing can be obtained by integration
of the non-adiabatic coupling matrix elements. Often, it is much easier to use an approximate
method, in which the mixing is determined by inspection of the CI coefficients of the MCSCF
or CI wavefunctions. This method is applicable only if the orbital mixing is negligible. For
CASSCF wavefunctions this can be achieved by maximizing the overlap of the active orbitals
with those of a reference geometry, at which the wavefunctions are assumed to be diabatic (e.g.
for symmetry reasons). The orbital overlap is maximized using using the new DIAB command
in the MCSCF program.