Comprehensive Basis-Set Testing of Extended Symmetry-Adapted Perturbation Theory and Assessment of Mixed-Basis Combinations to Reduce Cost

Hybrid or "extended" symmetry-adapted perturbation theory (XSAPT) replaces traditional SAPT's treatment of dispersion with better-performing alternatives, while at the same time extending two-body (dimer) SAPT to a many-body treatment of polarization using a self-consistent charge-embedding procedure. The present work presents a systematic study of how XSAPT interaction energies and energy components converge with respect to the choice of Gaussian basis set. Although errors can be reduced in a systematic way using correlation-consistent basis sets, similar performance at lower cost is obtained using Karlsruhe basis sets, and we introduce new versions with limited augmentation (diffuse functions) that are even more efficient. Pople-style basis sets, which are even more efficient, often afford good results if a large number of polarization functions are included. The dispersion models used in XSAPT afford much faster basis-set convergence as compared to the perturbative description of dispersion in conventional SAPT, meaning that "compromise" basis sets (such as jun-cc-pVDZ) are no longer required and benchmark-quality results can be obtained using basis sets of triple-zeta quality. The use of diffuse functions proves to be essential, especially for the description of hydrogen bonds. The "delta(Hartree-Fock)" correction that accounts for high-order induction can be performed in double-zeta basis sets without significant loss of accuracy, leading to a mixed-basis approach that offers 4x speedup over the existing (cubic-scaling) XSAPT approach.

Atmospheric Reaction of Hydrazine Plus Hydroxyl Radical. I. Reliable Pathways

Understanding the mechanism of hydrazine oxidation reaction by OH radical accompanied by the rate constants of all possible pathways is important. They are key parameters to explain the fate of hydrazine in the atmosphere. To reach the mentioned parameters, higher-level calculations by using quantum chemical methods have been implemented comprehensively for reliable channels such as H-abstraction, SN2, and addition/elimination reactions. To estimate the barrier energies of H-abstraction channels accurately, large numbers of the CCSD(T)/X calculations (where X denotes the augmented Dunning and Pople double zeta or triple zeta basis sets) have been applied to the optimized geometries of the MP2/aug-cc-pVTZ, MP2/maug-cc-pVTZ, and M062X/maug-cc-pVTZ levels. Contributions of excited states on the computed potential energy surface have been considered by the MR-MP2 (multi-reference) method in conjunction with the large augmented quadruple zeta, aug-cc-pVQZ, basis sets. The direct dynamic calculations have been carried out using the accurate energies of the CCSD(T) method and the partition functions of the second-order MØller-Plesset perturbation theory, and also by the validated M06-2X method with the aug-cc-pVTZ, and maug-cc-pVTZ basis sets. Finally, The VTST and TST theories have been used to calculate the temperature dependence of rate constants of the considered pathways. Also, the pressure-dependent rate constants of the barrierless pathways have been investigated by the strong collision master equation/RRKM theory.