Salles RAPHAEL - site d'Ivry-sur-Seine
Valence bond configuration interaction (VBCI) is introduced in this talk.
The method adds the dynamic correlation contribution by use of
configuration interaction (CI) after a VBSCF calculation. The VBCI method
allows various approaches with different structure selection procedures.
VBCIS involves only single excitations, while VBCISD involves also
doubles, and so on. VBCI(D,S) includes doubly excited structures for the
active electron-shell and singly excited structures for inactive pairs.
Using perturbation theory, VBCIPT truncates less important excitations and
estimates their contributions by an approximated perturbation formula. The
methods were tested by calculating the bond energies of H2, LiH, HF, HCl,
F2, and Cl2 as well as the barriers of identity hydrogen abstraction
reactions, X* + X'H --> XH + X'* (X, X ' = CH3, SiH3, GeH3, SnH3, PbH3).
Test calculation shows the VBCIS results are in a good agreement with
those of BOVB, while the VBCISD results match those of MO-based CCSD
method. The computational results of VBCI(D,S) and VBCIPT match those of
VBCISD with much less computational effort. As a demonstration of the
potential of the VBCI method, we calculated the barrier of the hydrogen
exchange reaction. The value of 10.0 kcal/mol of the VBCI barrier is in
very good agreement with the most accurate barrier of molecular orbital
based methods.