Showing papers by "Ernest R. Davidson published in 1974"

Configuration interaction calculations on the nitrogen molecule

Abstract: A contracted Gaussian basis set capable of describing about 63% of the correlation energy of N2 has been used in a detailed configuration-interaction calculation. Second-order perturbation theory overestimated the correlation energy by 23–50% depending on how H0 was chosen. Pair-pair interaction affects the correlation energy by about 20% while quadruple excitations have an 8% effect.

Configuration interaction description of electron correlation

Abstract: The fundamental goal of quantum chemistry is the development of a qualitatively correct set of concepts for describing the chemical and physical properties of molecules. When this goal is achieved, a well-trained chemist should be able to correlate and predict the behavior of most chemical systems without extensive calculation. In order to achieve this goal it is necessary to obtain high-accuracy wave functions for a few typical chemical systems. Further it is necessary to understand these wave functions on two levels. First one must understand what types of terms and effects must be included in the mathematical description of the wave function. This level of understanding has probably now been attained as a result of twenty years of large-scale calculations. On the second level one must understand the relationship between the physical and chemical properties of the constituent parts of the system and the physical and chemical properties exhibited by the system itself. This level of understanding has proven frustratingly difficult to achieve by the direct ab-initio approach.

Ab initio calculations on urea

Abstract: Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the 1A1(π π*) state. The 1A1(π π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π π* and π π* triplet states. The lowest singlet state is predicted to be the n π π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.

Matrix elements for spin‐adapted configurations

Abstract: It is shown that the turn-over rule which is used to simplify matrix elements between spin-projected Slater determinants may also be used to simplify formation of matrix elements between any orthonormal set of spin-coupled Slater determinants. The coefficients for the spin-coupling may then be chosen freely to reduce the number of important configurations in the secular equation.

Equivalence‐restricted open‐shell SCF theory

Abstract: A method is presented for generating open-shell equivalence-restricted SCF orbitals in high-symmetry situations using Roothaan–Hartree–Fock programs which are adapted for lower symmetry.