Preliminary Assessment of the First-Order Density Matrix in Quantum Monte Carlo from Density Matrix Theory.

The trial wave function commonly used in the quantum Monte Carlo method consists of the product of up-spin and down-spin Slater determinants, allowing accurate calculations of multielectronic properties, although it is not antisymmetric under the exchange of electrons with opposite spins. An alternative description that overcomes these limitations using the Nth-order density matrix was already presented. This study introduces two new strategies based on the Dirac-Fock density matrix for QMC that still fully preserve antisymmetry and electron indistinguishability. Simulations are performed for the ground and excited states of He, Li, and Be showing that the present formulation and the conventional separation of spins are appropriate for a correct description of these systems, except for singlet excited states of the He and Be atoms, and that a part of the antisymmetry (antiparallel spins) can be neglected.

Methanol, ethanol, propanol, and butanol adsorption on H-ZSM-5 zeolite: an ONIOM study.

The search for renewable raw materials less harmful to the environment, such as methanol, ethanol, 1-propanol, and 1-butanol has become attractive. These products are obtained more rapidly and efficiently by specific solid catalysts, mainly the zeolites. The Brønsted acid sites distributed over the sinusoidal and the straight channels are important for the alcohol dehydration reaction that produces widely used chemicals. Therefore, the ONIOM method was used to study methanol, ethanol, propanol, and butanol adsorption in H-ZSM-5 zeolite. PM6 and DFT levels were used for the high layer ONIOM, while the low layer was calculated using the UFF force field. DFT was calculated using the B3LYP global hybrid GGA, M06-2X hybrid meta-GGA, and the hybrid range separated ωB97X-D functionals at 6-31+G(d) basis set. The high layer ONIOM was completely relaxed. The binding energy shows dependence on the relaxed tetrahedra and position of acid site. The Si/Al ratio was also studied. Graphical Abstract HOMO orbital of adsorbed alcohols showing the main contribution of zeolite for small alcohols.

Rovibrational energy and spectroscopic constant calculations of CH4⋯CH4, CH4⋯H2O, CH4⋯CHF3, and H2O⋯CHF3 dimers.

In this work, we performed a thorough investigation of potential energy curves, rovibrational spectra, and spectroscopic constants for dimers whose interactions are mediated by hydrogen bonds and other hydrogen interactions. Particularly, we deal with CH4⋯CH4, CH4⋯H2O, CH4⋯CHF3, and H2O⋯CHF 3 dimers by employing accurate electronic energy calculations with two different basis sets at the MP2 level of theory. Following this, the discrete variable representation method was applied to solve the nuclear Schrödinger equation, thus obtaining spectroscopic constants and rovibrational spectra. The harmonic constant, ω e , presents a direct relation to the strength of dimer interactions. As a general rule, it was found that a decrease of interatomic distances is followed by the increase of D e for all dimers. This behavior suggests that the interaction of CH4⋯CH4 is the weakest among all dimers, followed by CH4⋯CHF3, CH4⋯H2O and the strongest interaction given by the H2O⋯CHF 3 dimer.

Theoretical study of CH4-CH4, CHF3-CH4, CH4-H2O, and CHF3-H2O dimers.

We have studied systems with typical hydrogen bonding and others with interaction involving hydrogen. CH(4)-CH(4), CH(4)-H(2)O, CHF(3)-CH(4), and CHF(3)-H(2)O dimers were studied using MPWB1K, PBE1PBE, MP2, and QCISD levels of theory with a large number of basis functions. The Pople 6-31+G(2d), 6-311++G(2d,2p), and 6-311++G(3df,3pd) as well as Dunning augmented aug-cc-pVDZ and aug-cc-pVTZ basis sets were used. The dimer geometries were fully optimized. An optimal basis set was determined for these systems to achieve a suitable compromise between accuracy and computational feasibility. A proper strategy was found for the electronic property calculations of dimers studied: the use of aug-cc-pVDZ as the optimal basis set at MP2 level. Dipole moments, polarizabilities, BSSE effects, and DeltaZPE were also analyzed for these dimers.