Hunting for Complex Organic Molecules in the Interstellar Medium: The Role of Accurate Low-Cost Theoretical Geometries and Rotational Constants

Abstract

A new approach to computation at affordable cost of accurate geometrical structures and rotational constants for medium-sized molecules in the gas phase is further improved and applied to a large panel of interstellar complex organic molecules. The most distinctive feature of the new model is the effective inclusion of core–valence correlation and vibrational averaging effects in the framework of density functional theory (DFT). In particular, a double-hybrid functional in conjunction with a quadruple-ζ valence/triple-ζ polarization basis set is employed for geometry optimizations, whereas a cheaper hybrid functional in conjunction with a split-valence basis set is used for the evaluation of vibrational corrections. A thorough benchmark based on a wide range of prototypical systems shows that the new scheme approaches the accuracy of state-of-the-art wave function methods with the computational cost of the standard methods (DFT or MP2) routinely employed in the interpretation of microwave spectra. Since the whole computational workflow involves the postprocessing of the output of standard electronic structure codes by a new freely available web utility, the way is paved for the accurate yet not prohibitively expensive study of medium- to large-sized molecules also by nonspecialists.

Quantum chemistry meets high-resolution spectroscopy for characterizing the molecular bricks of life in the gas-phase

“…All the geometry optimizations required by the PCS/DFT and PCS/bond models have been performed with the Gaussian package, 110 whereas the computation of the final PCS/bond geometries has been implemented using a website (https://www.skies-village.it/prox ima/pcsbonds/), which, starting from a given set of atomic numbers and Cartesian coordinates, generates interactively the corresponding PCS geometry and a 2D representation of the molecular structure. 111…”

Accurate Geometries of Large Molecules by Integration of the Pisa Composite Scheme and the Templating Synthon Approach

“…Previous studies , have shown that remarkably accurate geometrical parameters are delivered by the rev-DSD-PBEP86-D3BJ (hereafter rDSD) double-hybrid functional in conjunction with a basis set obtained from the s , p , d functions of the cc-pVTZ-F12 basis set and the f functions of the cc-pVTZ basis set. − Furthermore, d functions can be neglected for H atoms. Following the same philosophy, for atoms of the fourth and fifth row of the periodic table, small-core pseudopotentials are used in conjunction with a basis set in which the s , p , d functions are taken from the cc-pVQZ-PP basis set and the f functions from the cc-pVTZ-PP basis set. , The final basis set (referred to as 3F12 – ) is reported in the Supporting Information (SI) and has dimensions comparable with those of the jun-cc-pVTZ basis set employed systematically in previous studies, but delivers results much closer to those of augmented quadruple-ζ basis sets which are, in turn, close to the complete basis set (CBS) limit .…”

“…It has been shown that Δ r ij b can be expressed by a simple one-parameter function of the covalent radii and principal quantum numbers ( n ) of the involved atoms, together with the Pauling bond order ( P ij ): , normalΔ r i j b = A i j false| P i j − 2 + normalΔ O ( 1 − P i j ) false| with A i j = prefix− 0.0011 n i ′ n j ′ − 1 ( r Z i c o v + r Z j c o v ) and n k ′ = max {3, n k }. An additional term can be introduced for correcting the underestimation of C–F bond hyperconjugation (Δ hyp ): normalΔ r i j h y p = prefix− 0.025 ( P i j − 1 ) 2 normalΔ F …”

“…Based on these premises, we have developed and validated the Pisa composite scheme (PCS), ,, which improves the accuracy of current approaches for medium-sized molecules by nearly an order of magnitude, without any significant increase of computational cost, especially when an inexpensive one-parameter bond correction is employed (PCSB). ,, …”

“…Based on these premises, we have introduced and validated two models, which provide accurate molecular structures at the cost of geometry optimizations by a double hybrid functional in conjunction with a triple-ζ basis set followed by estimation of core–valence (CV) correlation effects by second-order Møller–Plesset perturbation theory (MP2) , or by a one-parameter empirical function . The accuracy of the approach can be further increased by correcting the tendency of several methods rooted in the density functional theory (DFT) to overestimate delocalization by a simple function of Pauling bond orders involving just one additional parameter. , The final model delivers an accuracy close to that of state-of-the-art composite wave function methods, ,− but can be routinely applied to much larger molecules. The purpose of the present paper is to further extend the model to molecules containing heavier elements (in particular halogen atoms) and to large, flexible molecules for which the accuracy of the approach is not fully satisfactory.…”