Gallium nitride (GaN) clusters are analyzed to set approaches for more extended calculations at the nanoscale. We test the atom components and very small clusters using the most sophisticated compound methods such as G1ÀG3, CBS, W1, and a flavor of DFT (B3PW91) using several sizes and qualities of basis sets. Results are compared with very precise experimental information when available. Interestingly, the B3PW91 yields results comparable to the high-quality compound methods. For negative ions, it is difficult to assess the quality of the methods; the electron affinity (EA) calculations, as expected, yield better results when diffuse functions are used. All ionization potentials (IPs) are well reproduced by all methods, but the best results are obtained with B3PW91 for the Ga atom and with the compound methods for N. Among the compound methods, the W1 ones use the largest basis sets with 93 functions (which include two sets of g-functions) for one gallium atom. Geometry optimizations are performed with MP2 for the G-methods, HF for the CBS-methods (except QB3), and B3LYP for the CBS-QB3 and all W1 methods. G3 followed by B3PW91 yields the best result for the Ga quartet excited state, and there is little difference for the excited doublet of N, among the compound methods. In general all calculated IPs and EAs are in good agreement with existing experimental data. We also performed extended calculations of bigger GaN clusters such as Ga x N y where 6 < x ≈ y < 19 using B3PW91/6-311G(d). Finally, we calculate second derivative and thus the vibrational spectra for all clusters studied. We generate harmonic force fields using the procedure Fuerza and averaged the force field parameters and use them in a simple heating molecular dynamics calculation that yields an acceptable value for the heat capacity.

We perform an ab initio analysis of the photoisomerization of the protonated Schiff base of retinal (PSB-retinal) from 11-cis to 11-trans rotating the C10-C11=C12-C13 dihedral angle from 0° (cis) to -180° (trans). We find that the retinal molecule shows the lowest rotational barrier (0.22 eV) when its charge state is zero as compared to the barrier for the protonated molecule which is ∼0.89 eV. We conclude that rotation most likely takes place in the excited state of the deprotonated retinal. The addition of a proton creates a much larger barrier implying a switching behavior of retinal that might be useful for several applications in molecular electronics. All conformations of the retinal compound absorb in the green region with small shifts following the dihedral angle rotation; however, the Schiff base of retinal (SB-retinal) at trans-conformation absorbs in the violet region. The rotation of the dihedral angle around the C11=C12 π-bond affects the absorption energy of the retinal and the binding energy of the SB-retinal with the proton at the N-Schiff; the binding energy is slightly lower at the trans-SB-retinal than at other conformations of the retinal.