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Papers by Martin Sulka

Journal of Computational Chemistry

We explore the possible route to approximate natural orbital occupation numbers‐based diagnostic ... more We explore the possible route to approximate natural orbital occupation numbers‐based diagnostic of differential multireference character of noncovalent energy differences by techniques based on many‐body expansion. It turns out that two‐body fragmentation of monomers may lead to a reasonable approximation of such a diagnostic in hydrogen‐bonded complexes. The results are useful, for example, for assessment of the expected bias cancellation in energy differences of larger systems obtained by single‐reference methods.

The Journal of Chemical Physics

Tributyl-phosphate (TBP), a ligand used in the PUREX liquid-liquid separation process of spent nu... more Tributyl-phosphate (TBP), a ligand used in the PUREX liquid-liquid separation process of spent nuclear fuel, can form explosive mixture in contact with nitric acid, that might lead to violent explosive thermal runaway. In the context of safety of a nuclear reprocessing plant facility, it is crucial to predict the stability of TBP at elevated temperatures. So far, only the enthalpies of formation of TBP is available in the literature with a rather large uncertainties, while those of its degradation products, di-(HDBP) and mono-(H2MBP) are unknown. In this goal, we have used state-of-the art quantum chemical methods to compute the formation enthalpies and entropies of TBP and its degradation products di-(HDBP), mono-(H2MBP) in gas and liquid phases. Comparisons of levels of quantum chemical theory revealed that there are significant effects of correlation on their electronic structures, pushing for the need of not only high level of electronic correlation treatment, namely local coupled cluster with single and double excitation operators and perturbative treatment of triple excitations [LCCSD(T)], but also extrapolations to the complete basis to produce reliable and accurate thermodynamics data. Solvation enthalpies were computed with the conductor like screening model for real solvents [COSMO-RS], for which we observe errors not exceeding 22 kJ mol −1. We thus propose with final uncertainty of about 20 kJ mol −1 standard enthalpies of

The Journal of Physical Chemistry A, 2014

We present a relativistic quantum chemical study to determine the best surrogate for plutonium(IV... more We present a relativistic quantum chemical study to determine the best surrogate for plutonium(IV) to be used in experimental investigations of the behavior of plutonium-nitrate-TBP in fire conditions that might occur in the nuclear fuel refining process known as PUREX. In this study geometries and stabilities of Pu(NO 3 ) 6 2− and Pu(NO 3 ) 4 (TBP) 2 complexes were compared to that of equivalent complexes of selected elements from the lanthanide and actinide series (Ce, Th, U) chosen on the basis of similar ionic radii and stability as tetravalent species. PBE and PBE0 DFT functionals have proven to be sufficient and affordable for qualitative studies, performing as good as the wave function based correlated method MP2. On the basis of our results, cerium(IV) appears to be a good surrogate for plutonium(IV).

Zeitschrift für Physikalische Chemie, 2013

We report the calculation of the H-, Br-, and I-abstraction channels in the reaction of OH radica... more We report the calculation of the H-, Br-, and I-abstraction channels in the reaction of OH radicals with bromoiodomethane CH 2 IBr. The resulting energy profiles at 0 K were obtained by high-level all-electron ab initio methods including valence and core-valence electron correlation, scalar relativistic effects, spin-orbit coupling, spin-adaptation, vibration contributions, and tunneling corrections. In terms of activation enthalpy at 0 K, the energy profile for the Br-abstraction showed that this reaction pathway is not energetically favorable in contrast to the two other channels (H-and I-abstractions), which are competitive. The H-abstraction was strongly exothermic (−84.4 kJ mol −1 ), while the I-abstraction was modestly endothermic (16.5 kJ mol −1 ). On the basis of our calculations, we predicted the rate constants using canonical transition state theory over the temperature range 250-500 K for each abstraction pathway. The overall rate constant at 298 K was estimated to be 3.40 × 10 −14 and 4.22 × 10 −14 cm 3 molecule −1 s −1 for complex and direct abstraction mechanisms, respectively. In addition, the overall rate constant computed at 277 K was used in the estimation of the atmospheric lifetime for CH 2 IBr. On the basis of our theoretical calculations, the atmospheric lifetime for the OH removal process is predicted to be close to 1 year. In terms of atmospheric lifetime, the OH reaction is not competitive with the Cl reaction and photolysis processes.

Chemical Physics Letters, 2013

We present the decomposition of interaction energies in beryllium clusters, Be m¼3À6 , into a ser... more We present the decomposition of interaction energies in beryllium clusters, Be m¼3À6 , into a series of nonidentical m-body nonadditivities up to m ¼ 6. We analyse the role of the electron correlation and compare nonadditivities at HF, CASSCF, MP2, CASPT2 and CCSD levels of theory with the reference aug-cc-pVTZ CCSD(T) results. The dominant nonadditivity terms are the three-and four-body. Five and six-body terms are relatively large in Be 5À6 clusters, but the total interaction energy of Be 6 is affected only marginally, due to their mutual cancellation.

The journal of physical chemistry. A, 2013

Reactants, weak molecular complexes, transition states, and products for the H-, Cl-, and I-abstr... more Reactants, weak molecular complexes, transition states, and products for the H-, Cl-, and I-abstraction channels in the reaction of OH radicals with chloroiodomethane CH 2 ICl as well as the energy profiles at 0 K have been determined using high-level allelectron ab initio methods. The results showed that all-electron DK-CCSD(T)/ANO-RCC approach performed very well in predicting the reactivity of iodine. In terms of activation enthalpy at 0 K, the energy profile for the Cl-abstraction showed that this reaction pathway is not energetically favorable in contrast to the two other channels (H-and I-abstractions), which are competitive. The H-abstraction was strongly exothermic (−87 kJ mol −1 ), while the I-abstraction was modestly endothermic (11.8 kJ mol −1 ). On the basis of our calculations including the following corrections to the potential energies: basis set saturation, valence and core−valence electron correlation, relativistic effects, spin-adaptation, vibration contributions, and tunneling corrections, rate constants were predicted using canonical transition state theory over the temperature range 250−500 K for each abstraction pathway. The overall rate constant at 298 K was estimated to be 4.29 × 10 −14 and 5.44 × 10 −14 cm 3 molecule −1 s −1 for complex and direct abstraction mechanisms, respectively. In addition, the overall rate constant computed at 277 K was used in the estimation of the atmospheric lifetime for CH 2 ICl. On the basis of our theoretical calculations, the atmospheric lifetime for the OH removal process is predicted to be close to 1 year. In terms of atmospheric lifetime, the OH reaction is not competitive with the Cl reaction and photolysis processes.

Journal of Physics B-atomic Molecular and Optical Physics, Jan 1, 2012

We present results of theoretical investigation of the stability of small beryllium clusters, Be ... more We present results of theoretical investigation of the stability of small beryllium clusters, Be 2−6 . Due to the fact that the clusters are homonuclear, their stability is sufficiently represented by the binding energy D e calculated per atom. The knowledge of this quantity offers a possibility of calculating more complex energetic effects, such as destruction or fusion of the beryllium clusters and adding or detaching a beryllium atom. The most stable structures among those of the singlet multiplicity were studied using the highly correlated coupled clusters method with iterative single-and double-excitations and the perturbative triple excitations (CCSD(T)). The effect of electron correlation, basis set saturation, basis set superposition error, relativistic effects and the core correlation were carefully investigated and discussed. The optimized virtual orbital space (OVOS) with the controlled accuracy technique is extensively applied to alleviate the computational cost of calculations. In this specific case, the use of the OVOS technique led to speed-ups in computational time by more than an order of magnitude. Finally, we present the basis set extrapolated binding energies with all of the aforementioned corrections properly included, which are the most reliable data for this key property for Be 3−6 clusters to date. Furthermore, these values can serve as a valuable benchmark for testing the accuracy of less time consuming computational methods applicable for larger beryllium clusters.

Chemical Physics Letters

Thermodynamic and kinetic parameters for the reaction HI + CH3 = I + CH4 are calculated using hig... more Thermodynamic and kinetic parameters for the reaction HI + CH3 = I + CH4 are calculated using high-level ab initio molecular orbital methods. Activation enthalpies at 0 K for the forward (0.33 kJ mol−1) and reverse (137.29 kJ mol−1) reactions are reported for the first time. Calculated reaction enthalpy ΔrH0K (−136.96 kJ mol−1) is in perfect agreement with its literature counterpart. Canonical transition state theory with an asymmetrical Eckart tunneling correction is used to predict the rate constants as a function of temperature (250–2500 K).► Thermodynamic and kinetic parameters are calculated using ab initio molecular orbital methods. ► Calculated reaction enthalpy at 0 K is in excellent agreement with its experimental counterpart. ► Rate constants are calculated from 250 to 2500 K and compared to the available literature data.

International Journal of Quantum Chemistry, 2008

The electron affinity (EA) of the oxygen molecule is calculated by the CCSD(T) method using the o... more The electron affinity (EA) of the oxygen molecule is calculated by the CCSD(T) method using the optimized virtual orbitals space (OVOS) technique by which the dimension of the original space of virtual orbitals can be significantly reduced. Extended basis sets, up to the doubly augmented correlation consistent d-aug-cc-pV6Z basis sets for O2 and O at their experimental geometries are used. We demonstrate that even when the space of virtual orbitals is reduced to 50% of the full space, the resulting EA of the O2 molecule is accurate to within 0.01 eV. At the same time, the computational effort is reduced by about an order of magnitude. With OVOS reduced to 60% of the full virtual space, results are almost accurate. Considering the complete basis set limit with so reduced OVOS, corrections for the core correlation, vibrational and relativistic effects, the electron affinity of O2 is 0.452 ± 0.01 eV. This value agrees very well with the full virtual orbital space, EA = 0.446 eV, and with the recent experimental value, EA = 0.448 ± 0.006 eV. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008