Electronic excitation energies of small ZniSi clusters - art. no. 013202

Improvements in the characterization of II-VI compound-based solar cells, a nd a recent experimental characterization of small clusters and nanoparticl es, make the study of small II-VI clusters Very interesting. In a previous work the global minima of small ZniSi clusters i = 1-9 were characterized. In order to calculate the excitation energies of these clusters, basically two methods are available: on the one hand, the traditionally used configur ation interaction singles (CIS) theory, and on the other hand the recently developed time dependent density-functional theory (TDDFT). Calculations of the excitation energies of small ZniSi clusters, i = 1 - 3 were performed with both methods in an attempt to find the most appropriate one. The relat ivistic compact effective core potentials and shared-exponent basis set of Stevens, Krauss, Basch and Jasien (SKBJ) [Can J. Chem. 70, 612 (1992)], sys tematically enlarged with extra functions, were used in this work. These la rger basis sets are labeled according to the number of added functions. Thu s, as an example. if two extra sp functions and one d function are added, t he final basis set is denoted SKBJ(2sp1d). These basis sets were combined w ith both methods. In this way the most appropriate method and basis-set com bination was chosen, for further excitation energy calculations on larger Z niSi clusters. It was seen in both methods that more than one polarization function was needed. Combined with the CIS method, the smallest basis yield ing good results was SKBJ(1sp2d3f), and with TDDFT SKBJ(1sp2d2f). In the CI S case, this basis was too large even for Zn3S3. In addition to this, in th e literature TDDFT was seen to provide a better description of the excitati ons, and therefore a TDDFT-SKBJ(1sp2d2f) combination was chosen for further calculations. However, due to the fact that no experimental data are avail able. some results confirming the TDDFT results are necessary, in this way ensuring that our choice is the correct one, Multireference configuration i nteraction calculations, combined with a triple-zeta double polarization (T Z2P) basis set, were carried out for ZniSi, i = 1 and 2. For i = 3, a TZ ba sis without polarization was used: otherwise the limit of 255 on the basis function number was exceeded. These results were clearly in agreement with the TDDFT results, and confirm our previous choice.