Jason M. Hughes

Ab initio structures and stabilities of helide cations

The electronic structure and molecular properties of helides of the form HeX (where X = B-Ne, Al-Ar and n = 1-3) were studied using the CCSD(T) method in conjunction with the series of correlation-consistent basis sets. The highest level of theory employed, the CCSD(T)−FC/cc-pVQZ model, was used to elucidate trends in bond lengths, dissociation energies and harmonic frequencies. The more highly charged species were found to have shorter bond lengths than the singly charged species. The ground states of the helide cations were often those with longer bond lengths when compared with the excited

Ab Initio investigation of the electronic structure and stabilities of triatomic helide ions: He2Xn+ clusters (where X=B–Ne, Al–Ar and n=1–2)

Abstract The electronic structure and molecular properties of helides of the form He 2 X n + (where X=B–Ne, Al–Ar and n =1–3) were studied using the CCSD(T) method in conjunction with the series of correlation-consistent basis sets of descending in quality from cc-pVQZ, aug-cc-pVTZ, cc-pVTZ to cc-pVDZ. The highest level of theory employed, the CCSD(T)_FC/cc-pVQZ model, was used to elucidate trends in bond lengths, dissociation energies and harmonic frequencies. The more highly charged species were found to have shorter bond lengths than the singly charged species. The ground states of the helide cations were often those with longer bond lengths when compared with the excited state ions.

Ab initio structures and stabilities of helide cations: HeXn+(X = B-Ne, Al-Ar and n = 1-3)

Abstract:The electronic structure and molecular properties of helides of the form (where , Al-Ar and ) were studied using the CCSD(T) method in conjunction with the series of correlation-consistent basis sets. The highest level of theory employed, the CCSD(T)-FC/cc-pVQZ model, was used to elucidate trends in bond lengths, dissociation energies and harmonic frequencies. The more highly charged species were found to have shorter bond lengths than the singly charged species. The ground states of the helide cations were often those with longer bond lengths when compared with the excited state ions.

AB INITIO INVESTIGATIONS OF THE ELECTRONIC STRUCTURE OF HECH+ AND HECH2+

Abstract The 1 A′ and 3 A″ electronic states of HeCH + and the 2 A′ ground electronic state of HeCH 2+ have been investigated using an all electron coupled cluster single, double and triple excitation [CCSD(T)] method coupled with an augmented correlation-consistent polarised core valence triple zeta basis set (aug-cc-pCVTZ). For the 1 A′ and 3 A″ electronic states of HeCH + , the CCSD(T)a aug-cc-pCVTZ model yielded an optimised geometry of C s symmetry eith a { r CHe , r CH , θ HCHe } structural parameters of {2.172 A, 1.129 A, 81.4°} and {1.675 A, 1.121 A, 101.4°} respectively. Moreover, this electronic model yielded the singlet state to be 26.3 kcal mol −1 lower in energy in energy when compared with the triplet state. For the 2 A′ ground electronic state of HeCH 2+ the optimised structural parameters were {1.388 A, 1.253 A, 110.5°}, respectively. The calculated harmonic frequencies for all these electronic states were found to be real.

ROVIBRATIONAL STATES OF THE 1A1 GROUND ELECTRONIC STATE OF SI3

The rovibrational spectrum of the ground electronic state (1A1) of Si3 has been calculated using QCISD(T)_FC electronic model, with a large contracted basis set. The three-dimensional ab initio discrete potential energy surface was fitted with a 6th order power series expansion, which incorporated a Dunham expansion variable. The χ2 for the fit was 6·7 x 10-4 au. The force field was embedded in the Eckart–Watson Hamiltonian in order to obtain variationally the low-lying vibrational and rovibrational wavefunctions and eigenenergies. The differences between the anharmonic and harmonic calculations for the fundamental modes v 1(a1),v 2(a1),v 3(b2) are 11, 2 and 20 cm-1respectively. Although the vibrational separations observed in photodetachment studies of Si3 - are 337 ± 10 cm-1 501 ± 10 cm-1, which are in close agreement with the calculated values of 343 cm-11 (two-quanta excitation in the bend mode) and 498 cm-1 (one-quantum excitation in the asymmetric stretch mode), respectively, it would be anticipated...

Ab initio investigations of the electronic structure of HeNH+ and HeNH 2+

Electronic structures of HeNH+ and HeNH2+ were investigated using the all-electron coupled cluster single, double and triple excitation (CCSD(T)lAE) method coupled with an augmented correlation-consistent polarised core valence triple zeta basis set (aug-cc-pCVTZ). For HeNH+ and HeNH2+, the CCSD(T)lAE/aug-cc-pCVTZ model yielded cations of Cs symmetry with 2 A″ and 1 A′ ground electronic states possessing optimised geometrical parameters {rN-H; rN-He; θH-N-He} of {1·064 A, 1·517 A, 88·5°} and {1·192 A, 1·309 A, 96·2°} respectively. Of all the linear isomeric forms possible for these two cations, the only converged minimum energy structures were for the 4Σ- states of HeHN+ and HeNH+ yielding optimised structural parameters of {1·093 A, 2·403 A, 180·0°} and {1·117 A, 1·364 A, 180·0°} respectively. These two linear structures were calculated to be 9·6 and 18·6 kJ mol-1 respectively above the Cs structure. The calculated harmonic frequencies were real for all these states and their magnitudes were sensitive to the incorporation of electron correlation.

Ab initio calculations of the rovibrational states of He2O2

Abstract The low-lying rovibrational states of the dihelium carbene dication, He2C2+, have been calculated using ab initio techniques. A 75-point potential energy surface was constructed using an all-electron coupled cluster single, double and triple excitation (CCSD (T)) method together with a correlation-consistent polarised core valence triple-zeta basis set. The CCSD(T) optimised geometry was of C2v symmetry with an RC-He bondlength of 1.570 A and a bond angle of 84.1 °. The discrete potential energy surface was fitted using a Pade (4, 5) power series expansion yielding a ( x 2 ) 1 2 of 8.2 × 1−6 a.u. The potential function was embedded in the Eckart-Watson Hamiltonian, which was solved variationally. The anharmonic fundamental frequencies for the breathe, bend and asymmetric stretch vibrations were calculated to be 683.1, 329.5, 623.8 cm−1 respectively. The low-lying rovibrational states were calculated variationally using a 560-configuration basis involving products of the vibrational eigenfunctions and plus and minus combinations of regular symmetric-top rotor functions.

Ab initio calculations of the rovibrational states of He2N2

Abstract The low-lying rovibrational states of the 2 B 1 ground electronic state of He 2 N 2+ were calculated using ab initio techniques. A 74 point potential energy surface was constructed using the CCSD(T) method coupled with a cc-pCVTZ basis set. The CCSD(T) optimised equilibrium geometry was of C 2v symmetry with an R NHe bond length of 1.329 A and a θ HeNHe bond angle of 88.6°. The discrete potential energy surface was fitted using a Pade (4,5) power series expansion, yielding a (χ 2 ) 1 2 of 9.98 × 10 −5 au. The potential function was embedded in the nuclear Schrodinger equation and solved variationally to yield the low-lying rovibrational states.

Ab initio vibration and ro-vibrational spectrum of the 1A1 state of He2Si2+

The low-lying ro-vibrational states for the ground electronic state (1A1) of HeSi2+ have been calculated using an ab initio variational solution of the nuclear Schrödinger equation. A 96 point CCSD(T)/cc-pCVQZ potential energy surface (PES) has been calculated and a Ogilvie-Padé (3,6) potential energy function has been generated. This force field was embedded in the Eckart-Watson Hamiltonian from which the vibrational and ro-vibrational eigenfunctions and eigenenergies have been variationally calculated. A 70 point QCISD/aug-cc-pCVTZ discrete dipole moment surface (DMS) was calculated and a 5th order power series expansion (in terms of the two bond lengths and the included bond angle) has been generated. Absolute line intensities have been calculated and are presented for some of the most intense transitions between the vibrational ground state and the low-lying ro-vibrational states of this ion.

Ab initio and analytical potential energy functions of K2Na

Abstract Ab initio variational calculations were performed on K 2 Na + at the MP4 (single, double and quadruple substitutions) level of theory using (20 s , 14 p , 2 d , 1 f ) [13 s , 8 p , 2 d , 1 f ] and (17 s , 11 p , 2 d , 1 f ) [10 s , 6 p , 2 d , 1 f ] basis sets for potassium and sodium respectively. The 1s, 2s and 2p orbitals on potassium, 1s orbital on sodium and their virtual counterparts were not included in the correlation treatment. A 63 discrete potential energy hypersurface was generated using a vibrational t -coordinate system. The minimum energy structure is of C 2v symmetry with a bond length and included bond angle of 3.855 A and 73.9° respectively. From this hypersurface analytical potential functions were obtained using power series expansions, which will be embedded in an Eckart-Watson rovibrational Hamiltonian and from which rovibrational eigenfunctions and eigenergies will be calculated.