Accurate Potential Energy Curves Research Articles

Mapping the Complete Reaction Path of a Complex Photochemical Reaction.

We probe the dynamics of dissociating CS_{2} molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.

ExoMol line lists XXV: a hot line list for silicon sulphide, SiS

SiS has long been observed in the circumstellar medium of the carbon-rich star IRC+10216 CW Leo. Comprehensive and accurate rotation-vibrational line lists and partition functions are computed for 12 isotopologues of silicon sulphide ($^{28}$Si$^{32}$S, $^{28}$Si$^{34}$S, $^{29}$Si$^{32}$S, $^{28}$Si$^{33}$S, $^{30}$Si$^{32}$S, $^{29}$Si$^{34}$S, $^{30}$Si$^{34}$S, $^{28}$Si$^{36}$S, $^{29}$Si$^{33}$S, $^{29}$Si$^{36}$S, $^{30}$Si$^{33}$S, $^{30}$Si$^{36}$S) in its ground (X $^1\Sigma^+$) electronic state. The calculations employ an existing spectroscopically accurate potential energy curve (PEC) derived from experimental measurements and a newly-computed ab initio dipole moment curve (DMC). The $^{28}$Si$^{32}$S line list includes 10104 states and 91715 transitions. These line lists are available from the ExoMol website www.exomol.com and the CDS database.

Charge transfer in low-energy collisions of H with He+ and H+ with He in excited states

The charge transfer process in collisions of excited (n = 2, 3) hydrogen atoms with He+ and in collisions of excited helium atoms with H+ is studied theoretically. A combination of a fully quantum-mechanical method and a semi-classical approach is employed to calculate the charge-exchange cross sections at collision energies from 0.1 eV u−1 up to 1 keV u−1. These methods are based on accurate ab initio potential energy curves and non-adiabatic couplings for the molecular ion HeH+. Charge transfer can occur either in singlet or in triplet states, and the differences between the singlet and triplet spin manifolds are discussed. The dependence of the cross section on the quantum numbers n and l of the initial state is demonstrated. The isotope effect on the charge transfer cross sections, arising at low collision energy when H is substituted by D or T, is investigated. Rate coefficients are calculated for all isotopes up to 106 K. Finally, the impact of the present calculations on models of laboratory plasmas is discussed.

Benchmark Linelists and Radiative Cooling Functions for LiH Isotopologues

Linelists and radiative cooling functions in the local thermodynamic equilibrium limit have been computed for the six most important isotopologues of lithium hydride, 7LiH, 6LiH, 7LiD, 6LiD, 7LiT, and 6LiT. The data are based on the most accurate dipole moment and potential energy curves presently available, the latter including adiabatic and leading relativistic corrections. Distance-dependent reduced vibrational masses are used to account for non-adiabatic corrections of the rovibrational energy levels. Even for 7LiH, for which linelists have been reported previously, the present linelist is more accurate. Among all isotopologues, 7LiH and 6LiH are the best coolants, as shown by the radiative cooling functions.

Accurate potential energy curves, spectroscopic parameters, transition dipole moments, and transition probabilities of 21 low–lying states of the CO+ cation

This paper calculates the potential energy curves of 21 Λ–S and 42 Ω states, which arise from the first two dissociation asymptotes of the CO+ cation. The calculations are conducted using the complete active space self–consistent field method, which is followed by the valence internally contracted multireference configuration interaction approach with the Davidson correction. To improve the reliability and accuracy of the potential energy curves, core–valence correlation and scalar relativistic corrections, as well as the extrapolation of potential energies to the complete basis set limit are taken into account. The spectroscopic parameters and vibrational levels are determined. The spin–orbit coupling effect on the spectroscopic parameters and vibrational levels is evaluated. To better study the transition probabilities, the transition dipole moments are computed. The Franck–Condon factors and Einstein coefficients of some emissions are calculated. The radiative lifetimes are determined for a number of vibrational levels of several states. The transitions between different Λ–S states are evaluated. Spectroscopic routines for observing these states are proposed. The spectroscopic parameters, vibrational levels, transition dipole moments, and transition probabilities reported in this paper can be considered to be very reliable and can be used as guidelines for detecting these states in an appropriate spectroscopy experiment, especially for the states that were very difficult to observe or were not detected in previous experiments.

Potential energy curves and transition properties for the ground and excited states of BH+ cation

BH+ cation is one of the candidates for laser cooling. The potential energy curves (PECs) for nine electronic states (X2+, A2, B2+, a4, b4+, 32+, 22, 32, 42+) relating to the B+(1Sg)+H(2Sg), B+(3Pu)+H(2Sg), B(2Pu)+H+(1Sg), and B+(1Pu)+H(2Sg) dissociation channels of BH+ cation are obtained using highly accurate multi-reference configuration interaction (MRCI) plus Davidson correction. All-electron basis sets AV5Z-DK for H and ACV5Z-DK for B are used in PEC calculations for the -i-S states of BH+ cation, respectively. In complete active space self-consistent field (CASSCF) calculation, H(1s2s2p3s3p) and B(2s2p) are chosen as active orbitals, B(1s) is the closed shell; in the MRCI calculation, the core-valence (CV) correction is considered, i.e., B(1s) shell is used for CV correlation. Spin-orbit coupling effects are considered with Breit-Pauli operators. Spectroscopic constants are fitted using the Murrell-Sorbie function. Spectroscopic constants for the X2+, A2, and B2+ states are in excellent agreement with the available experimental data; spectroscopic constants for the b4+, 32+, 32, and 42+ states are reported. Two potential wells for the 32 and 42+ states are found. The maximum fitting error of all electronic states is only 3.407 cm-1. In addition, PECs for the A2 and B2+ states are crossed at about 2.7 . Then, the transition dipole moments (TDMs) for the A2 X2+, B2+X2+, 32+X2+, B2+ A2, 32 X2+ and b4+ a4 transitions are also obtained. The strength for the B2+ A2 transition is very weak. Based on the accurate PECs and TDMs, the Franck-Condon factors and spontaneous radiative lifetimes are calculated. A strongly diagonal Franck-Condon factor (f00) for the A2X2+ transition is obtained, which equals 0.9414. Spontaneous radiative lifetime for the A2 and B2+ states is also predicted. i.e., (A2)=239.2 ns and (B2+)=431.2 ns. When SOC effect is considered, the A21/2 and B21/2+ states avoid crossing in the Franck-Condon region (R is about 2.7 ). Calculated f00 for the A21/2 X21/2+ transition is 0.9430; spontaneous radiative lifetime for the A21/2 is 239.0 ns. Our calculated results indicate that the influence for laser cooling BH+ cation via the crossing between B2+ and A2 states can be ignored.

A relativistic coupled-cluster interaction potential and rovibrational constants for the xenon dimer

ABSTRACT An accurate potential energy curve has been derived for the xenon dimer using state-of-the-art relativistic coupled-cluster theory up to quadruple excitations accounting for both basis set superposition and incompleteness errors. The data obtained is fitted to a computationally efficient extended Lennard-Jones potential form and to a modified Tang–Toennies potential function treating the short- and long-range part separately. The vibrational spectrum of Xe2 obtained from a numerical solution of the rovibrational Schrödinger equation and subsequently derived spectroscopic constants are in excellent agreement with experimental values. We further present solid-state calculations for xenon using a static many-body expansion up to fourth-order in the xenon interaction potential including dynamic effects within the Einstein approximation. Again we find very good agreement with the experimental (face-centred cubic) lattice constant and cohesive energy.

Accurate potential energy curve for helium dimer retrieved from viscosity coefficient data at very low temperatures

The long range potential of helium–helium interaction, which requires accurate ab initio calculation, due to the small value of the potential depth, approximately 11 K (9.5×10−4eV=0.091kJ∕mol) at 2.96 Å, will be obtained in this study by an alternative technique. This work presents a robust and consistent procedure that provides the long range potential directly from experimental data. However, it is difficult to obtain experimental data containing information regarding such a small potential depth. Thereby, sensitivity analysis will be used to circumvent this difficulty, from which viscosity data at lower temperatures (<5 K) were chosen as appropriate data to be used to retrieve the potential function between 3 and 4 Å. The linear relationship between the potential energy function and the viscosity coefficient will be established under quantum assumptions and the Bose–Einstein statistics. The use of quantum theory is essential, since the temperatures are below 5 K. The potential obtained in this study describes the viscosity with an average error of 1.7% that is less than the experimental error (5%), with the results being similar to those obtained for recent ab initio potentials.

A theoretical study of the dissociative recombination of SH+ with electrons through the 2Π states of SH.

A quantitative theoretical study of the dissociative recombination of SH+ with electrons has been carried out. Multireference, configuration interaction calculations were used to determine accurate potential energy curves for SH+ and SH. The block diagonalization method was used to disentangle strongly interacting SH valence and Rydberg states and to construct a diabatic Hamiltonian whose diagonal matrix elements provide the diabatic potential energy curves. The off-diagonal elements are related to the electronic valence-Rydberg couplings. Cross sections and rate coefficients for the dissociative recombination reaction were calculated with a stepwise version of the multichannel quantum defect theory, using the molecular data provided by the block diagonalization method. The calculated rates are compared with the most recent measurements performed on the ion Test Storage Ring (TSR) in Heidelberg, Germany.

A benchmark study of Li2+, Li2−, LiH+ and LiH−: Quantum Monte-Carlo and coupled-cluster computations

For the first time, very accurate potential energy curves (PEC) have been computed by the all electron Diffusion Quantum Monte-Carlo (DMC) and CCSD(T) methods for the ground-state of the Li2+, Li2−, LiH+ and LiH− ions. In addition to the molecular moieties, Li+, Li−, H− ions have also been studied by the aforementioned methods. The employed trial wave functions for the DMC calculations are a multiplication of multi-configuration expansions which are prepared by the Complete Active Space Self-Consistent Field method (CASSCF) with the aug-cc-pVTZ and the aug-cc-pCVTZ basis sets and the Jastrow factor. The CCSD(T) computations with all electrons correlated, have been performed by the cc-pV5Z, aug-cc-pCVQZ and aug-cc-pV6Z basis sets. In addition, the correlation energies for all of the studied atomic and molecular species have been calculated. For the studied molecular species, the Hartree-Fock energies in the complete basis set limit (ECBS) have not been reported. In this work for each of the molecular systems, the most negative energy which has been computed by the cc-pV5Z or aug-cc-pCVQZ basis sets has been used. The obtained PECs by the DMC and CCSD(T) methods are used to calculate the vibration-rotation energy levels and spectroscopic constants.

Coupled Cluster and Quantum Monte-Carlo potential energy curves of the ground state of Be2 and Be2+ molecules

Accurate potential energy curves (PECs) have been computed by the Diffusion Quantum Monte-Carlo (DMC) method for the ground state of the Be2 and Be2+ molecules. Multi-determinant trial wave functions (TWF) have been used in the DMC computations. The TWFs have been computed by the Complete Active Space Self-consistent Field (CASSCF) method with the aug-cc-pVTZ basis set. For atomic species (Be and Be+) the full active space and for molecular species (Be2 and Be2+) the valence full active space have been used. Despite using proper active spaces, the calculated binding energy is not fully compatible with the available experimental value. The source of this discrepancy has been investigated for the present and previous works. Furthermore, the PECs are recomputed by the CCSD(T) level of theory (with all electrons correlated) with the aug-cc-pCVQZ, cc-pV5Z, cc-pCV5Z, cc-pV6Z and 6ZaPa-NR basis sets. In addition, the correlation energy, the dissociation energy (De), the ionization potential (IP) and spectroscopic constants have also been computed by the obtained PECs. The Hartree-Fock (HF) energy has been computed by the aforementioned basis sets for all of the studied species. All of the obtained HF energies in this work are more negative than the previous computations, except for the Be atom.

Quantum Monte Carlo with density matrix: potential energy curve derived properties.

In this work, we used diffusion quantum Monte Carlo with density matrix (d-DMC) and variational quantum Monte Carlo (d-VMC) to determine the potential energy curve (PEC) and obtain the spectroscopic constants of H2 molecule in the ground state, in order to evaluate the capability of these methods to provide an accurate PEC description. These quantum Monte Carlo methods build with density matrix are new approaches to conventional quantum Monte Carlo methods based on wave function formed by product of α and β determinants. To investigate the robustness of d-DMC, we performed calculations with two different basis sets and analyzed the influence of the size of these sets on results. To the best of our knowledge, this is the first study that shows the dissociation energy and rotational constant obtained from d-QMC. We found that the quality of PEC described by the d-DMC is essentially coincident with the most accurate results available in the literature, regardless of the complexity of basis set employed.

Accurate MRCI calculations of the low-lying electronic states of the NCl molecule

Characterization of 22 electronic states of NCl correlating to the two lowest dissociation channels are carried out using high level CASSCF/MRCI calculations with a sextuple-ζ basis set including Davidson modification, core-valence correlation correction and scalar relativistic effects. As far as we know, this radical has never been the preference of theoretical researchers and experimental investigations only concern the ground state and the two low-lying metastable states. Accurate potential energy curves, dissociation energies as well as the equilibrium constants are determined and avoided crossings between the Π symmetry are studied. Moreover, spin-orbit splitting of several states and transition probabilities and radiative lifetimes for some allowed or forbidden transitions are presented.

Localized electron wave packet description of chemical bond and excitation: Floating and breathing Gaussian with valence-bond coupling

A model of localized electron wave packets (WPs) with variable position and width (floating and breathing) that are spin-coupled as per the valence-bond theory is presented. It produces accurate potential energy curves of LiH in the ground singlet and triplet states. Quantization in a mean-field approximation of the motion of a WP that corresponds to the Li 2s electron generates semi-quantitative potential energy curves of low energy excited states. Real-time semiquantal dynamics of the WP induced by an intense laser pulse gives high-harmonic generation spectra that capture qualitative features of a higher-level wave function calculation.

Accurate potential energy curves and spectroscopic properties of the 27 Λ-S states and 73 Ω states of the PO radical

ABSTRACTThe potential energy curves (PECs) were calculated for the 27 Λ-S states and 73 Ω states of PO radical. The calculations were done using the CASSCF method, which was followed by the internally contracted multireference configuration interaction (icMRCI) approach. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections as well as Davidson correction were included. Of the 27 Λ-S states, the 16Σ+ state was repulsive at any case. The 14Φ and 16Π states were bound, but they became repulsive with the spin-orbit coupling (SOC) effect accounted for. The 34Σ+, a4Π, C′2Δ, D′2Π, 14Δ, 12Φ, 16Σ+ and 16Π states were inverted with the SOC effect included. The F2Σ+ state had double wells. The avoided crossings existed between the B2Σ+ and F2Σ+ states, the F2Σ+ and 32Σ+ states, the C′2Δ and 22Δ states, the 14Δ and 24Δ states, the 24Δ and 34Δ states, the 24Π and 34Π states and the 34Π and 44Π states. The c4Σ+, 24Σ+, 34Σ+, 34Π, 44Π, 54Π, 34Δ, 14Φ and 16Π states were weakly bound, which well depths were within several hundred cm−1. The spectroscopic parameters were derived. The SOC effect on the spectroscopic properties was evaluated. The spectroscopic results obtained here could be expected to be reliably predicted ones.

Electronic structure of the polar molecules XF (X: Be, Mg, Ca) with rovibrational and dipole moment calculations

A theoretical investigation for the feasibility of laser-cooling is performed through the calculation of accurate potential energy curves, static dipole moments, spectroscopic constants and rovibrational calculations for 24, 26 and 27 highly excited electronic states for BeF, CaF and MgF molecules respectively. In order to understand the electronic structure of their lowest lying electronic states and to learn the characteristic behavior of their chemical bonding, a high level of calculation is realized by using the complete active space self-consistent field (CASSCF) with multi-reference configuration interaction MRCI method including single and double excitations with Davidson correction (+Q) for the three considered molecules. The comparison between the values of the present work and those available in the literature for several electronic states shows a good agreement. Fifty new excited electronic states have been investigated, in the present work, for the first time for the three studied molecules.

Accurate Ab Initio-Based Potential Energy Curve and Spectroscopic Properties of NO(X&lt;sup&gt;2&lt;/sup&gt;Π) via Extrapolation to the Complete Basis Set Limit

Accurate Ab Initio-Based Potential Energy Curve and Spectroscopic Properties of NO(X&lt;sup&gt;2&lt;/sup&gt;Π) via Extrapolation to the Complete Basis Set Limit

Calculating potential energy curves with fixed-node diffusion Monte Carlo: CO and N2.

This study reports on the prospect for the routine use of Quantum Monte Carlo (QMC) for the electronic structure problem, applying fixed-node Diffusion Monte Carlo (DMC) to generate highly accurate Born-Oppenheimer potential energy curves (PECs) for small molecular systems. The singlet ground electronic states of CO and N2 were used as test cases. The PECs obtained by DMC employing multiconfigurational trial wavefunctions were compared with those obtained by conventional high-accuracy electronic structure methods such as multireference configuration interaction and/or the best available empirical spectroscopic curves. The goal was to test whether a straightforward procedure using available QMC codes could be applied robustly and reliably. Results obtained with DMC codes were found to be in close agreement with the benchmark PECs, and the n3 scaling with the number of electrons (compared with n7 or worse for conventional high-accuracy quantum chemistry) could be advantageous depending on the system size. Due to a large pre-factor in the scaling, for the small systems tested here, it is currently still much more computationally intensive to compute PECs with QMC. Nevertheless, QMC algorithms are particularly well-suited to large-scale parallelization and are therefore likely to become more relevant for future massively parallel hardware architectures.

Ab initio calculations for the first-positive bands of N2

The absolute absorption spectral line intensities of the 0-0, 1-1, 2-2, 3-3, and 4-4 bands of N2(A3Σu+−B3Пg) are computed for the first time with the accurate potential energy curves (PECs) and transition dipole moment curves (TDMCs) using the ab initio calculation. During the calculation process, the precise multireference configuration interaction (MRCI) approach with the augmented correlation consistent polarized valence quadruple-zeta set (aug-cc-pVQZ) and active space 3110311 are used. The calculated PECs of the two states (A3Σu+, B3Пg) agree well with Rydberg-Klein-Rees (RKR) potential, and the calculated TDMCs are more reasonable than the results of other studies. Our calculated Einstein A coefficients are also well consistent with the recent experiment results. The intensities of five bands are obtained from the summation of their respective absolute line intensities, for instance, the 0-0 band intensities are 3.25×10−16cm−1/(moleculecm−2) at 300K and 1.65×10−16cm−1/(moleculecm−2) at 3000K. The theoretical results are credible. They are also beneficial for astrophysics where high temperature dominates.

Theoretical investigation of RbXe and CsXe excimers including the spin–orbit interaction

The potential energy curves and spectroscopic constants of ground and many excited states of the RbXe and CsXe van der Waals systems have been determined using a one-electron pseudopotential approach. This has reduced the number of active electrons of the RbXe and CsXe van der Waals systems to only one electron, the valence electron; and has led to the use of large basis sets for Rb, Cs and Xe atoms. In this context, the potential energy curves of the ground and many excited states are calculated at the SCF level, and the spin–orbit interaction is considered. The core–core interactions for Rb+Xe and Cs+Xe were included using the CCSD(T) accurate potential energy curves of Hickling et al (2004 Phys. Chem. Chem. Phys. 6 4233–4239). The spectroscopic constants of these states are derived and compared with the available theoretical and experimental results. Such comparisons show very good agreement for the ground and the first excited states. Moreover, the transition dipole function has been determined for a large and dense grid of internuclear distances, including the spin–orbit effect.