On the performance of composite schemes in determining equilibrium molecular structures

Abstract

Determination of equilibrium molecular structures is an essential ingredient in predicting spectroscopic parameters that help in identifying molecular carriers of microwave transitions. Here, the performance of two different ab initio composite approaches for obtaining equilibrium structures, “energy scheme” and “geometry scheme,” is explored and compared to semi‐experimental equilibrium structures. This study is performed for a set of 11 molecules which includes diatomics, linear triatomics, and a few non‐linear molecules. The ab initio calculations were performed using three tiers of composite chemical recipes. The current results show that as the overall rigor of calculation is increased, the semi‐experimental and the ab initio numbers agree to within 0.0003 Å for all molecules in the test set. The composite approach based on correcting the potential energy surface (energy scheme) and the one based on correcting the geometry directly (geometry scheme) show excellent agreement with each other. This work represents a step toward development of efficient and highly accurate procedures for computing ab initio equilibrium structures.

Toward Accurate Quantum Chemical Methods for Molecules of Increasing Dimension: The New Family of Pisa Composite Schemes

“…89 In general terms, the optimal level of accuracy associated with predicted rotational constants should be close to 0.1% (1 MHz for a constant of 1 GHz), which roughly corresponds to errors smaller than 0.001 Å for typical bond lengths and 0.002 radians (0.1°) for typical valence angles. 111 This target accuracy is reached for small systems by the most accurate composite schemes involving high excitation levels, 112 but larger molecules require less demanding approaches. Here we will show that the PCS2, MPCS1, and BPCS1 models are able to fulfill this task for molecules containing up to a dozen and a few dozen atoms, respectively.…”