Accurate Geometries\nof Large Molecules at DFT Cost\nby Semiexperimental and Coupled Cluster Templating Fragments

Accurate geometries of small semirigid\nmolecules in the\ngas phase\nare available thanks to high-resolution spectroscopy and accurate\nquantum chemical approaches. These results can be employed for validating\ncheaper low-level quantum chemical models or correcting the corresponding\nstructures of large molecules. On these grounds, in this work, a large\npanel of semiexperimental equilibrium structures already available\nin the literature is used to confirm the average error (1 mÅ\nfor bond lengths and 2 mrad for valence angles) of a version of the\nPisa composite schemes (PCS2), which is applicable to molecules containing\nup to about 20 atoms. Then, the geometries of 30 additional medium-sized\nsystems were optimized at the PCS2 level to cover a more balanced\nchemical space containing moieties poorly represented in SE compilations.\nThe final database is available on a public domain Web site (https://www.skies-village.it/databases/) and can be employed for correcting structures of larger molecules\nobtained by hybrid or double-hybrid density functionals in the framework\nof the templating molecule approach. Several examples show that corrections\nbased on the structures of building blocks taken from this database\nreduce the error of the B3LYP geometrical parameters of large molecules\nby nearly an order of magnitude without increasing the computational\ncost. Furthermore, the results of different density functional theory\n(DFT) or wave function (e.g., MP2) models can be improved in the same\nway by simply computing both the whole molecule and suitable building\nblocks at the chosen level. Then, whenever reference structures of\nsome building blocks containing up to about 20 atoms are not available,\nthey can be purposely optimized at the PCS2 level by employing reasonable\ncomputer resources. Therefore, a new DFT-cost tool is now available\nfor the accurate characterization of large molecules by experiment-oriented\nscientists.