Analytical second derivatives of the free energy in solution by the reference interaction site model self-consistent field explicitly including constrained spatial electron density distribution

Spin-Spin Coupling Constant Based on Reference Interaction Site Model Self-Consistent Field with Constrained Spatial Electron Density

A method for computing spin-spin coupling constants (SSCCs) using the reference interaction site model self-consistent field with constrained spatial electron density (RISM-SCF-cSED) is proposed for the first time. Describing solvents using integral equation theory allows us to reflect solvent effects at atomic resolution in SSCCs while accounting for thermal fluctuations at a low computational cost. Applying the method to water, 1,1-difluoroethylene, and 1-methylaminomethylene-2-naphthalenone revealed that the solvent shift was evaluated to a greater extent than in the continuum solvent model. The origin of this phenomenon was analyzed in terms of the physical mechanisms underlying SSCCs.

Vibrational Self-Consistent Field (VSCF) and Post-VSCF Method Calculations Combined with the Reference Interaction Site Model Self-Consistent Field Method Coupled with the Constrained Spatial Electron Density Distribution: Applications to NaHCOO in Aqueous Phase

Investigating vibrational behavior in solution is crucial for understanding molecular dynamics within a solvent environment. Notably, the analysis of Raman spectra for molecules in solution is important owing to its ability to unveil intricate solute-solvent interactions. Previous studies have effectively employed frequency calculations utilizing the reference interaction site model self-consistent field method in conjunction with constrained spatial electron density distribution (RISM-SCF-cSED) to understand molecular vibrations in solution, primarily focusing on fundamental vibrational modes. However, the oversight of overtones and combination tones in these studies prompted us to combine the vibrational self-consistent field (VSCF) and vibrational second-order Mo̷ller-Plesset perturbation (VMP2) methods with RISM-SCF-cSED to address these aspects theoretically. Illustrating the efficacy of this integrated approach, we computed the Raman spectra of sodium formate (NaHCOO) in water, revealing the necessity of accounting for molecular anharmonicity in solution vibrational analysis. Our findings underscore the potency of VSCF and VMP2 in conjunction with RISM-SCF-cSED as a robust theoretical framework for such calculations.

Lagrangian of extended multiconfigurational self-consistent field second-order quasidegenerate perturbation theory combined with reference interaction site model self-consistent field constraint spatial electron density

Lagrangians of the state-averaged multiconfigurational self-consistent field (SA-MCSCF) and multistate extended second-order quasidegenerate perturbation theory (MS-XMCQDPT2) coupled with the reference interaction site model self-consistent field constraint spatial electron density are defined. In addition, variational equations were derived to calculate the excitation energies of the target molecules dissolved in various solvents. The theory was applied to a phenol molecule in various solutions, and the gradients and Hessian matrices were calculated to evaluate the absorption spectral lines, including the broadening bandwidth. Numerical calculations revealed fine structures in any solvent surroundings. The main intramolecular vibrational modes related to such fine structures were stretching vibrations of the aromatic ring and the oxygen atom of the phenol molecule. The present theory plays an important role in predicting the structure of potential energy surfaces, such as Hessian matrices for various solvent types, during the photoexcitation process.

Recent developments and applications of reference interaction site model self-consistent field with constrained spatial electron density (RISM-SCF-cSED): A hybrid model of quantum chemistry and integral equation theory of molecular liquids

The significance of solvent effects in electronic structure calculations has long been noted, and various methods have been developed to consider this effect. The reference interaction site model self-consistent field with constrained spatial electron density (RISM-SCF-cSED) is a hybrid model that combines the integral equation theory of molecular liquids with quantum chemistry. This method can consider the statistically convergent solvent distribution at a significantly lower cost than molecular dynamics simulations. Because the RISM theory explicitly considers the solvent structure, it performs well for systems where hydrogen bonds are formed between the solute and solvent molecules, which is a challenge for continuum solvent models. Taking advantage of being founded on the variational principle, theoretical developments have been made in calculating various properties and incorporating electron correlation effects. In this review, we organize the theoretical aspects of RISM-SCF-cSED and its distinctions from other hybrid methods involving integral equation theories. Furthermore, we carefully present its progress in terms of theoretical developments and recent applications.

Theoretical Study of Raman Intensities of p-Nitroaniline in Different Solvent Conditions by Using a Reference Interaction Site Model Self-Consistent Field Explicitly Including Constrained Spatial Electron Density Distribution

Raman spectroscopy is one of the most powerful tools to understand and characterize the states and structures of systems in several environments. To obtain highly accurate changes in Raman intensities of systems in solution, theoretical treatment, which can deal with not only the states and structures of systems but also the environment around molecules, proves to be significant. Hence, in this study, we developed the calculation of changes in Raman intensities of systems in different solvent conditions by using the reference interaction site model self-consistent field study explicitly including constrained spatial electron density distribution; this model is designed based on elements from both quantum mechanics and statistical mechanics. We showed that our calculation method could reproduce the changes in Raman intensities of p-nitroaniline (pNA) under different solvent conditions, including supercritical water, which has been observed in previous experimental studies. Based on the analysis of the calculation results, we observed that the ratio of the Raman intensity change of pNA in different solvent conditions is strongly correlated with the charge-transfer character of pNA.

Theoretical Understanding of the Nonlinear Raman Shift of C≡N Stretching Vibration of p-Aminobenzonitrile in Supercritical Water

Subcritical and supercritical fluids (SCF) have attracted significant attention in the past few decades because of their unique properties. In a previous study, a nonlinear Raman shift of the C≡N stretching vibration of p-aminobenzonitrile (p-ABN) with respect to the supercritical water (SCW) density was observed [K. Osawa et al., J. Phys. Chem. A 2009, 113, 3143-3154]. Although a plausible mechanism of the nonlinear Raman shift was proposed in the study, the discussion at the atomistic level was inadequate. To elucidate the nonlinear Raman shift mechanism of the C≡N stretching vibration of p-ABN in SCW from a theoretical viewpoint, we employed RISM-SCF-cSED, which is the hybrid method between quantum mechanics and statistical mechanics. We discovered that the hydrogen-bonding effect is dominant at low- and middle-density regions, while the packing effect is dominant at the high-density region. The balances of these effects determine the Raman shift of p-ABN in SCF.

Investigation of the metastable structures of polyiodide in acetonitrile studied using global reaction route mapping and the reference interaction site model self-consistent field explicitly including constrained spatial electron density distribution

In this study, we theoretically analyzed the metastable structures of polyiodide (I₇^-) in the gas and acetonitrile phases using global reaction route mapping and the reference interaction site model self-consistent field explicitly including constrained spatial electron density distribution. From the chemical reaction pathways of I₇^- in acetonitrile, it was found that there would be 2 types of isomerization pathways. One proceeds with constant stoichiometry and the other takes place by breaking and forming I-I bonds. In addition, we discovered that I₇^- had various metastable structures within ∼10 kcal mol^-1. Comparing the most stable structure in the gas and acetonitrile phases, the tetrapot type is found to be the most stable structure in the gas phase; however, it is the zigzag type in acetonitrile. In order to understand this difference, we performed the decomposition analysis of the thermal correlation term in the gas and acetonitrile phases. It was found that thermal correction plays a key role in the stability and we could explain the difference in the population of the EQ states of I₇^- in each phase. Overall, we revealed that the solvation effect must be one of the crucial factors to stabilize the isomers of I₇^- and determine the chemical reaction pathways.