On the applicability of one- and many-electron quantum chemistry models for hydrated electron clusters

Abstract

We evaluate the applicability of a hierarchy of quantum models in characterizing the binding energy of excess electrons to water clusters. In particular, we calculate the vertical detachment energy of an excess electron from water cluster anions with methods that include one-electron pseudopotential calculations, density functional theory (DFT) based calculations, and ab initio quantum chemistry using MP2 and eom-EA-CCSD levels of theory. The examined clusters range from the smallest cluster size (n = 2) up to nearly nanosize clusters with n = 1000 molecules. The examined cluster configurations are extracted from mixed quantum-classical molecular dynamics trajectories of cluster anions with n = 1000 water molecules using two different one-electron pseudopotenial models. We find that while MP2 calculations with large diffuse basis set provide a reasonable description for the hydrated electron system, DFT methods should be used with 1 E-mail: turi@chem.elte.hu, fax: (36)-1-372-2592.3 precaution and only after careful benchmarking. Strictly tested one-electron psudopotentials can still be considered as reasonable alternatives to DFT methods, especially in large systems. The results of quantum chemistry calculations performed on configurations that represent possible excess electron binding motifs in the clusters appear to be consistent with the results using a cavity structure preferring one-electron pseudopotential for the hydrated electron, while they are in sharp disagreement with the structural predictions of a non-cavity model. 4

Excess electrons in methanol clusters: Beyond the one-electron picture

“…The small clusters were generated similarly to our previous aqueous analysis. 33 We determined the center of mass of the solvated electron using the one-electron pseudopotential, and produced clusters of gradually increasing size consisting of n methanol molecules (n = 2 -8) that are nearest to the center of mass of the electron. These clusters are thought to model the first (and part of the second) solvation shell of the excess electron.…”

“…This arrangement is similar to that seen in interior structure water cluster anions. 33 In surface state configurations dangling OH hydrogens localize the excess electron. We then performed various level quantum mechanical calculations on these clusters (100 configurations for each size), and calculated the energy difference between the anionic and the neutral species.…”

“…It was found that B3LYP and PBE0 perform better than BLYP, but BLYP still gives reasonable results. 52 Later studies indicated that even B3LYP and PBE0 overestimate electron binding 33 but using the BHandHLYP functional provides significantly better agreement with MP2 data. 29,30,33 It was pointed out that increasing the Hartree-Fock (HF) character in the exchange part of the functional improves the performance of DFT methods.…”

“…The aqueous case that has been thoroughly investigated highlights the essentially trivial statement that careful benchmarking is necessary to critically examine and characterize the applicability of the available methods. 29,30,31,32,33 It was found that a) at least MP2 level calculations and diffuse basis functions are required to reach reasonable description of the hydrated electron system; b) density functional theory based methods can be utilized only with sufficient care and after careful benchmarking, and c) carefully parameterized one-electron pseudopotentials may provide a semi-quantitative picture of the phenomenon and, thus, their application may be advantageous relative to DFT calculations, in particular, in large, non-reactive systems. 33 Clearly, similar treatment is necessary for methanol if one wants to go beyond the oneelectron-approach dictated simple picture.…”

“…Configurations of increasing size are generated using one-electron QCMD trajectories. This procedure is identical to that we applied in our hydrated electron studies, 33,34 and its aim is to generate realistic methanol configurations that model potential binding sites for the excess electron. Comparative calculations of the electron vertical detachment energy are carried out on the pre-formed binding sites of the cluster configurations that confine the electron in an interior solvent cavity, or bind the electron on the surface of the cluster.…”