Elastic Polarizable Environment Cluster Embedding Approach for Covalent Oxides and Zeolites Based on a Density Functional Method

Abstract

We present a new quantum mechanics/molecular mechanics (QM/MM) embedding approach for systems with directional polar covalent bonds. This “covalent elastic polarizable environment” (covEPE) scheme features a variational treatment of an energy expression that includes all degrees of freedom of both the QM region (the “cluster”) and the MM regions (the “environment”). The method completely and explicitly includes both the electrostatic and mechanical interactions between a QM model cluster and its environment. Monovalent pseudoatoms that represent real atoms of the material saturate the dangling bonds of the cluster; these pseudoatoms belong simultaneously to the QM and MM regions. For a correct description of a pure silica environment, we constructed a new force field of the shell-model type based on potential derived charges instead of formal charges. We implemented the covEPE approach in the density functional program ParaGauss and applied it to pure-silica and Al-containing chabazite, employing a generalized gradient approximation. These applications showed that calculated structural parameters and OH frequencies of bridging hydroxyl groups reproduce experimental data with good accuracy compared to other contemporary computational methods.

Computational Modelling of Nanoporous Materials

“…The model studies with covEPE method suggested that the DE energies of the acid sites decrease by about 20-25 kJ/mol along the series faujasite>CHA>HZSM-5. 19,20 This observation is in agreement with experimental trend in the acidity of these zeolites tentatively estimated from measured shifts of the OH frequency upon adsorption of a probe CO molecule. 51 The presence of additional metal cation in vicinity to the bridging OH groups has substantial influence on the Brnsted acidity.…”

“…Three different hybrid QM/MM methods based on summation scheme have been developed in the last years for zeolites and similar systems: method included in ChemShell software, 32,33 the scheme developed by Sulimov et al 18 , and covEPE method. [19][20][21] A hybrid MP2/DFT method was proposed by Tuma and Sauer 22 for theoretical simulation of problems that involve both bond rearrangements and van-der-Waals interactions. This method combines second-order Mller-Plesset perturbation theory (MP2) as high-level method for the reaction site with DFT as low-level method for the large part of the zeolite system under periodic boundary conditions.…”

The heterogeneity of the hydroxyl groups in chabazite

“…18 The relative stability of the acidic protons is in agreement with other cluster calculations using 13T atoms 41 and an embedded 5T cluster. 42 In periodic calculations 20,37,39,41 the H 1 proton position becomes more stable. As can be seen from Table I the proton affinities of the oxygen atoms show a strong dependence on the choice of the cluster.…”

“…Compared to our results, the difference in the proton affinity of the two oxygen atoms is larger, 17 kJ/mol, and in their calculations the oxygen atom O 1 has a higher proton affinity than the oxygen atom O 3 . Using a different embedding method Naslusov et al 42 have found the proton affinity of the O 3 oxygen atom to be higher by 6 kJ/mol than that of the O 1 oxygen atom. 26,39 and embedded 2T clusters.…”

Density Functional Embedded Cluster Study of Cu4, Ag4 and Au4 Species Interacting with Oxygen Vacancies on the MgO(001) Surface

“…[10,11] These methods allow one to describe accurately adsorption on metal-oxide surfaces of varying degrees of covalence while taking relaxation effects into account. We employed EPE embedding for ionic oxides to elucidate the adsorption of isolated Pd atoms on regular sites and oxygen vacancies of MgOA C H T U N G T R E N N U N G (001).…”