Ignacio Nebot-Gil

Towards an accurate molecular orbital theory for excited states: Ethene, butadiene, and hexatriene

A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans-1,3-butadiene, and trans-trans-1,3,5-hexat ...

THE CHEMICAL BONDS IN CUH, CU2, NIH AND NI2 STUDIED WITH MULTICONFIGURATIONAL SECOND ORDER PERTURBATION THEORY

The performance of multiconfigurational second order perturbation theory has been analyzed for the description of the bonding in CuH, Cu2, NiH, and Ni2. Large basis sets based on atomic natural orbitals (ANOS) were employed. The effects of enlarging the active space and including the core‐valence correlation contributions have also been analyzed. Spectroscopic constants have been computed for the corresponding ground state. The Ni2 molecule has been found to have a 0+g ground state with a computed dissociation energy of 2.10 eV, exp. 2.09 eV, and a bond distance of 2.23 A. The dipole moments of NiH and CuH are computed to be 2.34 (exp. 2.4±0.1) and 2.66 D, respectively.

Self‐consistent intermediate Hamiltonians: A coupled cluster type formulation of the singles and doubles configuration interaction matrix dressing

This paper presents a new self‐consistent dressing of a singles and doubles configuration interaction matrix which insures size‐consistency, separability into closed‐shell subsystems if localized molecular orbitals (MOs) are used, and which includes all fourth order corrections. This method yields, among several schemes, a reformulation of the coupled cluster method, including fully the cluster operators of single and double excitations, and partially those of the triples (Bartlett’s algorithm named CCSDT‐1a). Further improvement can be easily included by adding exclusion principle violating corrections. Since it leads to a matrix diagonalization, the method behaves correctly in case of near degeneracies between the reference determinant and some doubles. Due to its flexibility this formulation offers the possibility of consistent combination with less expensive treatments for the study of very large systems.

A CI study of the CuCO and CuCO+ complexes

MO CI calculations are carried out using an optimal space of valence virtual MOs obtained by means of a projection technique, as a linear combination of the AOs which are more occupied in the molecular Fock space. Localization of the occupied MOs and nonvalence virtual MOs is also achieved. The overall procedure is proven to be quite advantageous and well suited to obtain potential energy curves which keep the same physical meaning along the range of distances studied. Using a slightly better than double‐zeta quality basis set, a valence CAS‐CI, and selected CI wave function by the CIPSI algorithm have revealed a possible weak van der Waals interaction for the 2Σ+ state of CuCO, which remains when polarization functions are added to the basis set for the carbon and oxygen atoms. Even though the CuCO 2Π and CuCO+ 1Σ+ states are energetically close, the nature of the interactions is quite different, π bonding and mainly electrostatic, respectively. The results give further support to the view of the neutral metal–CO interaction as a balance of σ repulsion and π backbonding. However, it is proposed that the driving force for the positive ion metal–CO interaction becomes essentially electrostatic.

Theoretical study of the low‐lying states of trans‐1,3‐butadiene

We present extensive ab initio calculations on the low‐lying electronic states of trans‐1,3‐butadiene within the multireference configuration interaction (MRCI) framework by selecting the configurations with a perturbative criterion. The X 1Ag ground state and 1 3Bu, 1 3Ag, 2 1Ag, and 1 1Bu valence excited states have been calculated at a fixed geometry. The results obtained are in good agreement with previous experimental and calculated values, and could help to understand polyene spectroscopy, photochemistry, and photophysics. The advantages of a MRCI method where the most important contributions to the total MRCI wave function, perturbatively selected, are treated variationally, and the remaining terms are evaluated by means of a perturbational approach, are also discussed. Furthermore, a criterion in order to build a correlation‐consistent configuration interaction space is stated and, therefore, a reliable approximation to achieve accurate energy differences is obtained. Several monoelectronic molecu...

Universal model for the calculation of all organic solvent–water partition coefficients

Abstract We present the basis for building a universal organic solvation model to calculate solubility in any organic solvent and in water, as well as the organic solvent–water partition coefficient ( P ). Log P values are of the same order of magnitude as reference calculations but for a few cases which are discussed. Normalized log P contributions are sensitive to the rest of the atoms. When comparing porphin with phthalocyanine, the latter results in an amphipathic molecule. For C 70 , the contribution of a–e carbons to log P correlates with the distances from the nearest pentagon. The method has been also applied to benzobisthiazole oligomers and phenyl alcohols.

Applicability of size-consistent self-consistent configuration interaction to excited states

Abstract The size-consistent self-consistent configuration interaction (SC) 2 CI method is applied to the calculation of excitation energies. This method diagonalizes a dressed SDCI matrix built for the ground state but it is shown that the extra eigenvalues are not phony solutions but represent the excited roots of the system. Numerical tests are encouraging. The method is further improved by using the (SC) 2 MRCI scheme where the desired excited determinant is added to the ground state determinant as a generator of the CI space. Excellent results are obtained with this method for single and double replacement excitations in CH 2 .

Ab initio study of the mechanism of the atmospheric reaction: NO2 + O3 → NO3 + O2

The atmospheric reaction NO2 + O3 → NO3 + O2 (1) has been investigated theoretically by using the MP2, G2, G2Q, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. The results show that the reaction pathway can be divided in two different parts at the MP2 level of theory. At this level, the mechanism proceeds along two transition states (TS1 and TS2) separated by an intermediate, designated as A. However, when the single‐reference higher correlated QCISD methodology has been employed, the minimum A and the transition state TS2 are not found on the hypersurface of potential energy, which confirms a direct reaction mechanism. Single‐reference high correlated and multiconfigurational methods consistently predict the barrier height of reaction (1) to be within the range 2.5–6.1 kcal mol−1, in reasonable agreement with experimental data. The calculated reaction enthalpy is −24.6 kcal mol−1 and the reaction rate calculated at the highest CASPT2 level, of k = 6.9 × 10−18 cm3 molecule−1 s−1. Both results can be regarded also as accurate predictions of the methodology employed in this article.

Interacting induced dipoles polarization model for molecular polarizabilities. Reference molecules, amino acids and model peptides

Abstract We outline a method for the calculation of molecular dipole ( μ ) and quadrupole ( Θ = ) moments and dipole–dipole polarizabilities ( α = ) which we have successfully applied to a series of reference molecules, amino acids and model peptides. The results for μ are in line with CPHF reference calculations. In particular, the calculated positive value of CO is in agreement with both experimental and CI calculations. The computation of ( α = ) has been performed by the interacting induced dipoles polarization model that calculates tensor effective anisotropic point polarizabilities (method of Applequist et al.). The POLAR program cannot be used as a black box. Some tests should be performed when a new molecule is calculated. The POLAR program was designed for large molecules. Although in some large molecules the POLAR program has been successfully applied to predict trends, the test with small molecules shows that we have to touch up the approximations along the formulation. The results for ( α = ) of reference molecules are shown better for the POLAR program than for the PAPID algorithm. In the former case, the POLAR-AP model is revealed superior to the POLAR-IP method. The results for the amino acids and model peptides show that, as a rule, the PAPID program produces the better results, while the POLAR program produces acceptable results and converges in all the cases. In general, POLAR-IP underestimates the molecular polarizabilities and anisotropies while POLAR-AP overestimates these properties.

Size-consistent single-reference methods for electronic correlation: a unified formulation through intermediate hamiltonian theory

AbstractUsing the intermediate hamiltonian theory as a unique conceptual frame and the technique of CI matrix dressing, a wide series of single-reference methods for the treatment of the ground state correlation are reviewed, compared, and sometimes improved. These methods range from independent excitation approximation (the very next step beyond MP2) to coupled cluster, going through the so-called electron pair approximations and the (SC)2CI formalism. A hierarchy of these methods can be established according to two criteria:1.The physical effects incorporated in the model space, the choice of which is flexible.2.The quality of the evaluation of the coefficients of the external space determinants. This evaluation, which remains based on a single reference expansion of the wave function, may simply ensure the size consistency or incorporate the linked contributions from the outer space. These formulations in terms of diagonalizations of dressed CI matrices avoid convergence problems, but their main advantage is their flexibility, since they apply to multi-reference SDCI spaces as well as to SDCI spaces. The use of a common frame allows one to propose consistent combinations of methods of various costs for the treatment of various parts of the correlation energy.