Accurate Structures and Spectroscopic Parameters of Guanine Tautomers in the Gas Phase by the Pisa Conventional and Explicitly Correlated Composite Schemes (PCS and PCS-F12)

Abstract

A general strategy for the accurate computation of structural and spectroscopic properties of biomolecule building blocks in the gas phase is proposed and validated for tautomeric equilibria. The main features of the new model are the inclusion of core-valence correlation in geometry optimizations by a double hybrid functional and the systematic use of wave-function composite methods in conjunction with cc-pVnZ-F12 basis sets with separate extrapolation of MP2 and post-MP2 contributions. The resulting Pisa composite scheme employing conventional (PCS) or explicitly correlated (PCS-F12) approaches is applied to the challenging problem of guanine tautomers in the gas phase. The results are in remarkable agreement with the experimental structures, relative stabilities, and spectroscopic signatures of different tautomers. The accuracy of the results obtained at reasonable cost by means of black-box parameter-free approaches paves the way toward systematic investigations of other molecular bricks of life also by non-specialists.

Accurate Thermochemical and Kinetic Parameters at Affordable Cost by Means of the Pisa Composite Scheme (PCS)

“…Furthermore, thanks to the use of the 3F12 basis set and two-point extrapolation, conventional computations provide results close to those of their explicitly correlated counterparts without the need for any empirical parameter. In this connection, together with the results presented in the previous sections, a recent study of guanine tautomers showed that PCS results are very close to the best available values, which were obtained at the W1–F12 level …”

“…A cheaper alternative (PCS/DFT) is obtained by replacing the CCSD(T)/3F12 contribution with its revDSD-PBEP86-D3(BJ)/3F12 counterpart − (this combination of functional and basis set will be referred to in the following simply as rDSD). In this case, recent work has shown that CBS extrapolation can be neglected so that r ( PCS / DFT ) = r normalr D S D + normalΔ r CV 2 with normalΔ r CV 2 = r ( ae ‐ MP 2 / wC 3 ) − r ( fc ‐ MP 2 / wC 3 ) This version is particularly effective since low-scaling methods or analytical implementations are widely available for MP2 (or double-hybrid density functionals) gradients and several recent studies have confirmed that PCS/DFT geometries are sufficiently accurate for most applications. ,, Concerning open-shell systems, it has been shown that the opposite behavior of UHF and UMP2 contributions with respect to spin contamination leads to very stable results when using double-hybrid functionals . As a consequence, rDSD gradients and Hessians have always been computed by employing the unrestricted formalism.…”

“…Benchmark computations have been performed by an explicitly correlated version of the method (PCS-F12), which is obtained by replacing conventional MP2 and CCSD­(T) steps with their MP2-F12 and CCSD­(T)-F12 , counterparts in the E V 2 and Δ E V terms. In particular, the CCSD­(F12*) model of Hattig and co-workers was employed in conjunction with the size-consistent (T+) correction for the triple excitations .…”

“…Finally, the experimental values of spin−orbit couplings are employed for O, OH, SH, and Cl radicals, lowering their electronic energies by 0.9, 0.8, 2.3, and 3.5 kJ mol −1 , respectively. 30 Benchmark computations have been performed by an explicitly correlated version of the method (PCS-F12), 31 which is obtained by replacing conventional MP2 and CCSD(T) steps with their MP2-F12 32 and CCSD(T)-F12 33,34 counterparts in the E V2 and ΔE V terms. In particular, the CCSD(F12*) model of Hattig and co-workers was employed 35 in conjunction with the size-consistent (T+) correction for the triple excitations.…”

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

“…Recently, the LR and TM approaches have been combined with remarkable success in the nano-LEGO model , (also called LEGO-brick , ). However, the use of a large number of parameters remains quite unsatisfactory and, above all, suitable fragments are not always available. , Based on these premises, it is the purpose of this paper to show that much better results can be obtained by the recent 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 core–valence (CV) correlation is accounted for by a simple one-parameter approach (PCS/Bonds). We will consider in particular polyaromatic hydrocarbons (PAHs), together with some heteroaromatic derivatives (PANHs) and CN-substituted species .…”