Ab Initio

I nterest in the chemistry of the early actinide elements (notably uranium through americium) usually results either from the nuclear waste problem or the unique chemistry of these elements that result from 5f contributions to bonding. Computational actinide chemistry provides one useful tool for studying these processes.

The lowest energy electronic transitions in the weakly bound van der Waals complex of water and oxygen (H 2 O•O 2 ) are studied using ab initio methods. The vertical excitation energies for the two low-lying singlet states are calculated with the complete active space self-consistent field and multireference configuration interaction ͑MRCI͒ methods, and are compared to those calculated in the oxygen molecule. The MRCI calculations predict blueshifts of about 150 and 250 cm Ϫ1 for the transition frequencies on formation of the complex. These calculated shifts can provide assistance towards the spectroscopic identification of H 2 O•O 2 .

We studied Structure -Activity Relationships (SAR) of a series of 2,3,4,4a,5,9b-hexahydroindeno[1,2-c]pyridine derivatives, which present antispermatogenic activity, using Principal Component Analysis (PCA), K-Nearest Neighbor (KNN) and Neural Network (NN) methods. Firstly, we carried out thorough conformational explorations using the PM3 method, in order to determine the most stable conformations. The minima found were optimized using ab initio HF/3-21G. Molecular properties were calculated using the same ab initio method. Steric properties such as molecular volume and surface and the octanol/water partition coefficient were also calculated. A total of three variables (atomic charges on two specific atoms and the molecular volume) out of many were selected as being useful for SAR. PCA yielded a reasonable separation between high-activity and low-activity groups. The KNN and NN methods presented approximately 70% correct classifications. Finally, a set of new compounds was proposed and analyzed. q

Using the Unrestricted Hartree-Fock method and supercells containing up to 160 atoms, we calculated the energy level positions in the gap and atomic geometry for the Fe4+impurity substituting for a host Ti atom in SrTiO3. In agreement with experiment, the high spin (S=2) state is much lower in energy than the zero-spin state. The energy level positions strongly depend on the asymmetric displacement mode of the six nearest O ions which is a combination of the Jahn-Teller and breathing modes. A considerable covalent bonding between the Fe ion and four nearest O ions takes place.

"En este trabajo realizamos cálculos de primeros principios para investigar las propiedades estructurales y electrónicas de la multicapa 1x1 CrN/GaN. Los cálculos se realizan en las fases zincblenda y wurtzita, debido a que este es el estado base del nitruro de cromo CrN y el nitruro de galio GaN, respectivamente. Sin embargo, se estudia la estabilidad de la multicapa en la fase NaCl, con el fin predecir posibles transiciones de fase. Encontramos que la fase más favorable para multicapa, es la hexagonal tipo wurtzita, con posibilidad de pasar a la fase NaCl mediante la aplicación de una presión externa. Nuestros cálculos nos permiten predecir que la presión de transición es 13,5 GPa. A partir de la densidad de estados encontramos que la multicapa posee un comportamiento metálico debido a la hibridación de los orbitales Cr-d y N-p que atraviesan el nivel de Fermi"