Watrous Alexandria G, Davis Megan C, Fortenberry Ryan C
Department of Chemistry & Biochemistry, University of Mississippi, University, Mississippi 38677-1848, United States.
J Phys Chem A. 2024 Mar 21;128(11):2150-2161. doi: 10.1021/acs.jpca.3c08168. Epub 2024 Mar 11.
Highly accurate anharmonic vibrational frequencies of electronically excited states are not as easily computed as their ground electronic state counterparts, but recently developed approximate triple excited state methods may be changing that. One emerging excited state method is equation of motion coupled cluster theory at the singles and doubles level with perturbative triples computed via the (a)* formalism, EOMEE-CCSD(T)(a). One of the most employed means for the ready computation of vibrational anharmonic frequencies for ground electronic states is second-order vibrational perturbation theory (VPT2), a theory based on quartic force fields (QFFs),fourth-order Taylor series expansions of the potential portion of the internuclear Watson Hamiltonian. The combination of these two is herein benchmarked for its performance for use as a means of computing rovibrational spectra of electronically excited states. Specifically, the EOMEE-CCSD(T)(a) approach employing a complete basis set extrapolation along with core electron inclusion and relativity (the so-called "CcCR" approach) defining the QFF produces anharmonic fundamental vibrational frequencies within 2.83%, on the average, of reported gas-phase experimentally assigned values for the test set including the states of HCF, HCCl, HSiF, HNO, and HPO. However, some states have exceptional accuracy in the fundamentals, most notably for ν of HCCl in which the CcCR QFF value is within 1.8 cm at 927.9 cm (or 0.2%) of the experiment. Additionally, this approach produces rotational constants to, on the absolute average, within 0.41% of available experimental data, showcasing notable accuracy in the computation of rovibronic spectral data. Furthermore, utilizing a hybrid approach composed of harmonic CcCR force constants along with a set of simple EOMEE-CCSD(T)(a)*/aug-cc-pVQZ QFF cubic and quartic force constants is faster than using pure CcCR and better represents those modes that suffer from numerical instability in the anharmonic portion of the QFF, implying that this so-called "CcCR + QZ" QFF approach may be the best for future applications. Finally, complete, rovibrational spectral data are provided for :CCH, a molecule of potential astrochemical interest, in order to aid in its potential future experimental rovibronic characterization.
电子激发态的高精度非谐振动频率不像其基态对应物那样容易计算，但最近开发的近似三激发态方法可能正在改变这种情况。一种新兴的激发态方法是单双激发水平的运动方程耦合簇理论，通过(a)形式计算微扰三激发，即EOMEE - CCSD(T)(a)。对于基态电子态，最常用的计算振动非谐频率的方法之一是二阶振动微扰理论（VPT2），这是一种基于四次力场（QFF）的理论，是核间沃森哈密顿量势能部分的四阶泰勒级数展开。本文对这两者的组合用作计算电子激发态振转光谱的方法的性能进行了基准测试。具体而言，采用完全基组外推以及包含核心电子和相对论（所谓的“CcCR”方法）来定义QFF的EOMEE - CCSD(T)(a)方法，对于包括HCF、HCCl、HSiF、HNO和HPO的 态的测试集，其非谐基本振动频率平均在报道的气相实验指定值的2.83%以内。然而，有些态在基频上具有极高的精度，最显著的是HCCl的ν，其中CcCR QFF值在927.9 cm处与实验值相差1.8 cm（或0.2%）。此外，这种方法产生的转动常数在绝对平均值上，与可用实验数据相差在0.41%以内，在振转光谱数据计算中显示出显著的精度。此外，使用由谐波CcCR力常数以及一组简单的EOMEE - CCSD(T)(a)/aug - cc - pVQZ QFF三次和四次力常数组成的混合方法比使用纯CcCR更快，并且能更好地表示那些在QFF非谐部分存在数值不稳定性的模式，这意味着这种所谓的“CcCR + QZ”QFF方法可能是未来应用的最佳选择。最后，为具有潜在天体化学研究价值的:CCH提供了完整的振转光谱数据，以帮助其未来可能的实验振转光谱表征。
