F12+EOM Quartic Force Fields for Rovibrational Predictions of Electronically Excited States

Abstract

Quartic force fields (QFFs) constructed using a sum of ground-state CCSD(T)-F12b energies with EOM-CCSD excitation energies are proposed for computation of spectroscopic properties of electronically excited states. This is dubbed the F12+EOM approach and is shown to provide similar accuracy to previous methodologies at lower computational cost. Using explicitly correlated F12 approaches instead of canonical CCSD(T), as in the corresponding (T)+EOM approach, allows for 70-fold improvement in computational time. The mean percent difference between the two methods for anharmonic vibrational frequencies is only 0.10%. A similar approach is also developed herein which accounts for core correlation and scalar relativistic effects, named F12cCR+EOM. The F12+EOM and F12cCR+EOM approaches both match to within 2.5% mean absolute error of experimental fundamental frequencies. These new methods should help in clarifying astronomical spectra by assigning features to vibronic and vibrational transitions of small astromolecules when such data are not available experimentally.

DFT + F12 QFFs for Cost-Effective Rovibrational Spectral Data Predictions of Ground and Excited Electronic States

“…DFT + F12 QFFs are utilized in this work to efficiently compute the anharmonic vibrational frequencies and rotational constants for both ground state and electronically excited states of a test set of molecules chosen for their available reference data utilized in previous work. , The F12 portion for ground electronic states is built upon the F12-TcCR approach based on CCSD­(T)-F12b at a triple-ζ basis level (“T”) along with the corresponding core correlation cc-pCVTZ-F12 basis set (“cC”). ,, For further accuracy improvements, the Douglas–Kroll formalism within canonical CCSD­(T) is implemented as a composite term within the single-point energy to account for scalar relativistic effects (“ R ”). Again, the F12-TcCR + EOM approach is a composite method that relies upon CCSD­(T)-F12b energies for the reference, ground-state energies, and EOM-CCSD for the excitation energies.…”

“…Several composite methods have been proposed in our group of late to address this issue. − These most often utilize EOM-CCSD for the excited-state computation but are conjoined to some CCSD­(T) definition of the ground electronic state. While full EOM-CCSDT methods with considerations for complete basis set (CBS) extrapolations (“C”) and core electron correlation (“cC”) provide accurate descriptions in the so-called EOM-CCSDT-CcC approach, they are prohibitively time-consuming for all but the smallest molecules.…”

“…While full EOM-CCSDT methods with considerations for complete basis set (CBS) extrapolations (“C”) and core electron correlation (“cC”) provide accurate descriptions in the so-called EOM-CCSDT-CcC approach, they are prohibitively time-consuming for all but the smallest molecules. The use of explicitly correlated approaches in CCSD­(T)-F12b in the ground electronic state combined with EOM-CCSD for the excited state with the addition of scalar relativity (“ R ”) to the CcC considerations shows promise in the F12-TcCR + EOM methodology, which reports mean absolute percent differences (MA%Ds) compared to gas phase experiment of less than 2.5% . Here, the CBS extrapolation is not utilized as the basis is truncated simply at the triple-ζ level (“T”) due to the faster CBS convergence of F12 .…”

Performance of EOM-CCSD(T)(a)*-Based Quartic Force Fields in Computing Fundamental, Anharmonic Vibrational Frequencies of Molecular Electronically Excited States with Application to the Ã1A″ State of :CCH2 (Vinylidene)

“…Some have been based on EOM-CCSD with corrections for triples in EOM-CC3 corrections. , More recently, highly accurate, CCSD­(T) ground state energies corrected with EOM-CCSD excitation energies treated in various ways have produced mixed success for computing QFFs . Inclusion of explicit correlation from the F12 formalism , in the ground state term provides the same level of confidence in the fundamental vibrational frequency predictions but for less computational time. , The use of the ionization potential version of EOM in EOMIP-CCSDT is rigorously the best choice among those tested thus far for computing QFFs, but the scaling of the method and size of the basis sets make these computations impossibly time-consuming save for the smallest molecules or the most efficient supercomputers . Hence, new wave function-based, excited state methods need to be identified in order for application to QFFs if means beyond vibronic variational computations are to be identified.…”

“…EOM-CCSD­(T)­(a)* is then able to transform EOM-CCSD­(T)­(a) into an method through its unique treatment of the triples, which spans both the ground and excited state correlation energies . Hence, this method will certainly be less costly than EOM-CCSDT and will be utilized here in its electronically excited (EE) format: EOMEE-CCSD­(T)­(a)*. As such, its performance for use as a means of computing rovibrational spectra of electronically excited states (effectively rovibronic spectral data) for the simple QFF VPT2 approach will be tested in this work building off of a related, previous study .…”