
 Monday, January 1, 1996
 A Fifthorder Multiperturbation Derivation of the Energy Coefficients of Polyatomic Molecules
 Published at:AnNajah J. Res, Vol. 4, No.10 (1996)
A multiperturbation theory has been developed for molecular systems. In the present paper we extend this theory to fifth order in the energy. The "barenucleus" hydrogenic function is chosen as the zeroorder wave function rather than the more customary hartreefock function. With this choice the multiperturbation wave functions are independent of the nuclear charges and of the total number of nuclear centers and electrons for the molecule and are thus completely transferable to other systems. Making the simplest possible choice, we describe an nelectron, mcenter polyatomic molecule as n "hydrogenic" electrons on a single center perturbed by electronelectron and electronnucleus coulomb interactions. With this choice of zeroorder Hamiltonian (H0) the firstorder wave function for any polyatomic molecule will consist entirely of twoelectron, onecenter and oneelectron, twocenter firstorder wave functions. These are exactly transferable from calculations on Helike and H2like systems. To calculate the firstorder and second order correction for the wave function of any polyatomic molecule, we need the firstorder and secondorder correction for a twoelectron atomic wave function, the firstorder and secondorder correction for a oneelectron diatomic molecular wave function and some additional mixed secondorder corrections. The wave functions necessary will be twocenter, oneelectron at most.
The secondorder wave function for a polyatomic molecule contains additional contributions which cannot be obtained from the simple subsystems, but represent multiple perturbation contributions which are two electron diatomic, and oneelectron triatomic in character.
The expressions for the multiperturbation energyexpansion coefficients through fifth order are derived.
