Unbiased Comparison between Theoretical and Experimental Molecular Structures and Properties: Toward an Accurate Reduced-Cost Evaluation of Vibrational Contributions

Abstract

The tremendous development of hardware and software is constantly increasing the role of quantum chemical (QC) computations in the assignment and interpretation of experimental results. However, an unbiased comparison between theory and experiment requires the proper account of vibrational averaging effects. In particular, high-resolution spectra in the gas phase are now available for molecules containing up to about 50 atoms, which are too large for a brute-force approach with the available QC methods of sufficient accuracy. In the present paper, we introduce hybrid approaches, which allow the accurate evaluation of vibrational averaging effects for molecules of this size beyond the harmonic approximation, with special attention being devoted to rotational constants. After the validation of new tools for relatively small molecules, the β-estradiol hormone and a prototypical molecular motor have been considered to witness the feasibility of accurate computations for large molecules.

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

“…Coming to ZPEs and entropies, the VPT2 route offers a remarkable cost/performance ratio in conjunction with PCS1 harmonic frequencies and PCS0 anharmonic contributions. Thanks to the availability of very effective engines employing finite differences of analytical Hessians, these computations can be performed for quite large molecules. , Alternatively, the much faster methods based on analytical gradients in conjunction with scaled PCS1 or, even, PCS0 harmonic frequencies usually provide sufficiently accurate results.…”

The Semiexperimental Approach at Work: Equilibrium Structure of Radical Species

“…Both quantum-chemical programs compute the harmonic force field using analytical second-derivative techniques (in a normal coordinates representation) and the full set of cubic force constants by numerical differentiation. Noted is that for large molecules, the second and semidiagonal third derivatives needed for the evaluation of vibrational corrections can be obtained by a much cheaper approach employing analytical gradients. , …”