FOURFOLD CLUSTERS OF ROVIBRATIONAL ENERGIES IN H2PO STUDIED WITH AN AB-INITIO POTENTIAL-ENERGY FUNCTION

We report here an ab initio investigation of the cluster effect (i.e., the formation of four-member groups of nearly degenerate rotation-vib ration energy levels at higher J and K-a values) in the H2Po molecule. The potential energy function has been calculated ab initio for a tot al of 143 molecular geometries by means of the CCSD(T) method, using a n averaged relativistic effective potential for Po in conjunction with a newly optimized basis set. The values of the potential energy funct ion obtained cover the region up to around 5000 cm(-1) above the equil ibrium energy. On the basis of the ab initio potential, the rotation-v ibration energies of (H2Po)-Po-209 have been calculated with the MORBI D (Morse oscillator rigid bender internal dynamics) Hamiltonian and co mputer program. In particular, we have calculated the rotational energ y manifolds for J less than or equal to 40 in the vibrational ground s tate. We find that the formation of fourfold clusters in H2Po is very similar to that in H2Te. (C) 1997 Academic Press.