The potential energy surface of isomerising disilyne.

A (semi-)global, analytical potential energy surface is reported for the ground electronic state of the isomerising disilyne molecule, Si2H2. The surface reproduces well ab initio energies calculated at the CCSD(T) level with a cc-pV(Q+d)Z basis set for over 50 000 symmetrically unique molecular geometries. Of these ab initio points, 33 000 were used in a least-squares fit to determine the parameters of the analytical surface and the remainder to provide an independent test/validation set. The fitted surface includes: the four known isomeric forms of disilyne, dibridged, monobridged, disilavinylidene and trans-bent; the three most important transition states and four other critical points. The surface reproduces accurately existing experimental spectroscopic data for the dibridged and monobridged isomers and predictions are made for the disilavinylidene and trans-bent forms. The surface has the correct symmetry properties with respect to permutation of like atoms and is suitable for detailed dynamics studies of the isomerising Si2H2 system. Also reported is a systematic investigation of the critical points using the CCSD(T) and MRCI methods and basis sets up to 6-zeta quality: the effects of core-correlation, augmentation with diffuse functions and tight-d functions have been studied. The basis sets include the correlation consistent core-valence, cc-pCV(n+d)Z, basis sets recently developed by Yockel and Wilson [Theor. Chem. Acc., 2008, 120, 119]. Very good agreement is obtained between the theoretical and experimental equilibrium geometries, rotational constants and three available vibration frequencies for the dibridged isomer and for the rotational constants of the monobridged isomer. Multireference character, as measured by the T1 diagnostic, is found to vary significantly across the 12 critical points investigated.