Transition probability approach for direct calculation of coefficients of Configuration Interaction wave function

To reduce the computation cost of Configuration Interaction (CI) method, a new technique is used to calculate the coefficients of doubly excited determinants directly from orbital energies, orbital overlap matrix and electron population obtained from Hartree Fock level run. This approach to approximate the coefficients of CI wave function is termed as transition probability approximated CI (TPA-CI). In principle, calculated dynamical electron correlation energy of TPA-CI and Full CI (FCI) are equivalent. It is observed that computed TPA-CI correlation energies of hydrogen, water, ammonia and ozone are very close to FCI values, within 5% error. The potential energy curve of the hydrogen molecule is also studied and it is found that the energy is minimum at its equilibrium bond length.