Vibrational analysis of the ubiquitous interstellar molecule cyclopropenylidene (c-C3H2): the importance of numerical stability

Abstract

Recent work on cyclopropenylidene, c-CH, sought to improve on past ab initio results with a complete basis set (CBS) quartic force field method capable of predicting fundamental bands to within 5 cm−1 accuracy of gas-phase spectroscopy experiments for semirigid molecules. It was suggested that a problematic coupling between n-body correlation and 1-particle basis set effects prevented a proper application of this method for the out-of-plane bending vibrations of c-CH. We establish here that the actual source of this anomalous behaviour is numerical instability in the out-of-plane bending quartic force constants. It is found that the use of the normal equations to solve for the quartic force field, as well as a bending coordinate grid which is too compact, leads to errors of ca. 20 cm−1 in and . Our revised analysis shows and to be 980 cm−1 and 777 cm−1, respectively.