Ab initio near edge soft x-ray absorption fine structure (AI-NEXAFS) spectrum of ethylene

We present here an ab initio calculation of the C(1s(-1),pi*) absorption ba nd of ethylene. The linear vibronic coupling model is used with the aid of large configuration interaction (CI) ab initio calculations. The vibronic m odel involves the three totally symmetric vibrational motions (nu(1),nu(2), nu(3)) and the two non-totally symmetric ones (nu(11),nu(12)). For all thes e in-plane modes, the ab initio potential energy curves are obtained for th e electronic ground state and for the two lowest core-excited (1s -->pi*) s tates by multireference CI calculations including variational and pertubati ve contributions. The potential curves are fitted by polynomial expansions from which two sets of effective parameters are extracted for the linear co upling model. In this way, through the effective parameters, anharmonicitie s of the potentials and changes in the frequency from the ground to the cor e-excited states are taken into account without any adjustment. The contrib ution of the out-of-plane modes (nu(4),nu(7),nu(8)) is also considered assu ming the commutation of in-plane and out-of-plane Hamiltonians. The final c alculated spectrum involving the in-plane and the out-of-plane vibrational modes is found to be in good agreement with the experimental one, showing e vidence for symmetry breaking and core-hole localization. (C) 1999 American Institute of Physics. [S0021-9606(99)30546-8].