Ernst-Albrecht Reinsch

The self‐consistent electron pairs method for multiconfiguration reference state functions

An efficient direct CI method which includes all singly and doubly substituted configurations with respect to an arbitrary multiconfiguration (MCSCF) reference function is described. The configurations are generated by subsequently applying spin‐coupled two‐particle annihilation and creation operators to the complete MCSCF function. This considerably reduces the size of the n‐electron basis and the computational effort as compared to previous multireference CI treatments, in which the configurations are defined with respect to the individual reference configurations. The formalism of the method is very similar to the closed‐shell ’’self‐consistent electron pairs’’ (SCEP) method of Meyer. The vector Hc is obtained in terms of simple matrix operations involving coefficient and integral matrices. A full transformation of the two‐electron integrals is not required. Test calculations with large basis sets have been performed for the 3B1 and 1A1 states of CH2 (ΔE = 9.5 kcal/mol) and for the CH2(3B1) +H2→CH3+H r...

Molecular properties from MCSCF‐SCEP wave functions. I. Accurate dipole moment functions of OH, OH−, and OH+

Potential energy and dipole moment functions for the ground states of OH, OH−, and OH+ have been calculated from MCSCF, MCSCF‐SCEP, and SCEP‐CEPA electronic wave functions. The stability of the dipole moments with respect to the number of configurations (up to 598) and orbitals (up to 14) simultaneously optimized in the MCSCF procedure and the number of reference configurations (up to 11) in the MCSCF‐SCEP wave functions (up to 69 830 configurations) has been investigated. The dipole moment functions obtained from the best electronic wave functions are more accurate than all previously calculated ones. The deficiencies of the former calculations have been critically analyzed. The OH− and OH+ ions are predicted to be stronger IR emitters than the neutral OH radical. The rotationless rates of spontaneous emission A10 for the fundamental transitions are calculated to be 12.2, 137, and 263 s−1 for OH, OH−, and OH+, respectively. The calculated dipole moments in the vibrational ground states are 1.65, 1.04, an...

Accurate ab initio calculations of radiative transition probabilities between the A 3Σ+u, B 3Πg, W 3Δu, B′ 3Σ−u, and C 3Πu states of N2

Potential energy and electronic transition moment functions for the lowest five triplet states of N2 have been calculated from highly correlated multiconfiguration‐reference CI wave functions. From the calculated transition moments and RKR potential energy functions, radiative transition probabilities and lifetimes have been evaluated which are believed to be accurate within 15%. The theoretical lifetime of 36.7 ns of the C 3Πu state is in close agreement with the most recent experimental values. The calculated lifetimes of the B 3Πg state decrease from 13.4 μs for v′=0 to 4.82 μs for v′=12. The values for v′=5 to v′=12 are in good agreement with recent LIF measurements. The lifetimes for v′=0 to v′=4, however, are considerably larger than most previously measured values. The fact that the calculated values are very stable with respect to improvements of the electronic wave functions leads us to conclude that the experimental values are too low. Empirical transition moment functions for the B–A system are...

Ab initio calculations of radiative transition probabilities in SH, SH+, and SH−

Potential energy and dipole moment functions for the ground states of SH, SH+, and SH− have been calculated from highly correlated electronic wave functions. The electric dipole moments in the vibrational ground states of 32SH, 32SH+, and 32SH− are calculated to be 0.74, 1.29, and 0.27 D, and the rotationless rates of spontaneous emission  A10 to be 1,  52, and 75 s−1, respectively. The predicted transition probabilities between the low lying vibrational states of the electronic ground state of SH and SD are among the smallest so far known for dipole allowed rotation‐vibration transitions. The calculated A–X transition probabilities in SH confirm recent indirect determinations of the radiative lifetimes and absorption oscillator strengths in the predissociating v’=0 level of the A state. The 4Σ− state is calculated to intersect the A 2Σ+ state at R=3.1 a.u., between the classical turning points of v’=0 and 1 in the A state.

Spectroscopic properties of the hydroxonium ion calculated from scep cepa wavefunctions

Abstract Vibrational states involving symmetric stretching and symmetric bending motion have been calculated for H 3 O + from a SCEP CEPA potential energy surface. A very strong infrared transition with an absorption band strength of 1035 cm −2 atm −1 at 298 K is predicted at 559 cm −1 and further strong bands are calculated at 961 and 356 cm −1 .

Calculation of dynamic polarizabilities of He, H2, Ne, HF, H2O, NH3, and CH4 with MC‐SCF wave functions

In order to calculate frequency dependent polarizabilities, time dependent perturbation theory for MC‐SCF wave functions has been used and a significant part of the correlation contribution is accounted for. Explicit formulas for the coefficients of the system of linear equations to be solved have been given. The results reported for α(ω) of He, H2, Ne, HF, H2O, NH3, and CH4 show that in the case of the two electron systems excellent results can be obtained and that in the case of the ten electron systems with 45‐configuration wave functions about 80% of the correlation contribution for ω=0 have been included. The absolute accuracy for the latter case is in the range of 4% to 9.5%. For large ω values the experimental polarizability increases slightly more than the calculated values.

Theoretical integrated vibrational band intensities of water vapor

Using variational rotational–vibrational wave functions and ab initio electric dipole moment functions, rotational–vibrational line strengths and integrated vibrational band intensities of water vapor have been calculated. The theoretical line strengths are in excellent agreement with existing experimental data.

Ab initio calculation of potential energy surfaces and spectroscopic properties of H2S and H3S

Potential energy surfaces and spectroscopic properties were calculated for H2S and H3S+ from highly correlated SCEP‐CEPA wave functions. The equilibrium geometry of H3S+ is predicted to be re =1.350 A and θe =32.2°. The vibrational frequencies of H323S+ (in cm−1) were calculated to be 2529 (ν1), 1050 (ν2), 2527 (ν3), and 1208 (ν4) which are all in close agreement with experimental values obtained for solid H3S+SbF−6. The computed proton affinity for H2S of PA298=716.7 kJ mol−1 is in very good agreement with experiment.

Ab initio calculation of the dipole moment function of hydrogen iodide

Abstract SCEP/CEPA and MC SCF potential energy and dipole moment functions for hydrogen iodide have been calculated. Spectroscopic constants and vibrational dipole matrix elements obtained from the CEPA functions are in good agreement with experimental data. In contrast to previous results for hydrogen fluoride, the MC SCF dipole moment function is less accurate than the CEPA function.

Theoretical potential energy function and rovibronic spectrum of CO+2(X 2Πg)

For the electronic ground state of CO+2 the three‐dimensional potential energy, electric dipole, and transition moment functions have been calculated from highly correlated multireference configuration interaction electronic wave functions. Along the antisymmetric stretching displacements the shape of the potential energy functions is found to be very sensitive to the electron correlation effect. Using a modified theoretical potential energy function rovibronic energy levels have been calculated variationally by the method of Carter and Handy. In this approach, anharmonicity, rotation–vibration, electronic angular momenta, and electron spin coupling effects have been accounted for. The vibronic band origins agree to within about 10 to 20 cm−1 with the available experimental data, and the rotational levels agree to within 0.01 cm−1 for low J values. Additional vibrational band origins have been predicted for energies up to 3200 cm−1. The anomalously low frequency of the antisymmetric stretching mode and it...