Investigations of spectroscopic parameters and molecular constants for 1 ＋ X^1 ∑g^＋, W^3△u, and W1△u electronic states of P2 molecule

The potential energy curves （PECs） of three low-lying electronic states （X～1Σg～+,w～3△u,and W～1△u） of P2 molecule are investigated using the full valence complete active space self-consistent field （CASSCF） method followed by the highly accurate valence internally contracted multireference configuration interaction （MRCI） approach in conjunction with the correlation-consistent basis set in the valence range.The PECs of the electronic states involved are modified by the Davidson correction and extrapolated to the complete basis set （CBS） limit.With these PECs,the spectroscopic parameters of the three electronic states are determined and compared in detail with the experimental data.The comparison shows that excellent agreement exists between the present results and the available experimental data.The complete vibrational states are computed for the w～3△u and W～1△u electronic states when the rotational quantum number J equals zero and the vibrational level G（v）,the inertial rotation constant Bv,and the centrifugal distortion constant Dv of the first 30 vibrational states are reported,which accord well with the experimental data.The present results show that the two-point extrapolation scheme can obviously improve the quality of spectroscopic parameters and molecular constants.