Polarized Triple-zeta Basis Set Research Articles

Ab initio determination of potential energy surfaces for the first two UV absorption bands of SO2

Three-dimensional potential energy surfaces for the two lowest singlet (Ã(1)B1 and B̃(1)A2) and two lowest triplet (ã(3)B1 and b̃(3)A2) states of SO2 have been determined at the Davidson corrected internally contracted multi-reference configuration interaction level with the augmented correlation-consistent polarized triple-zeta basis set (icMRCI+Q∕AVTZ). The non-adiabatically coupled singlet states, which are responsible for the complex Clements bands of the B band, are expressed in a 2 × 2 quasi-diabatic representation. The triplet state potential energy surfaces, which are responsible for the weak A band, were constructed in the adiabatic representation. The absorption spectrum spanning both the A and B bands, which is calculated with a three-state non-adiabatic coupled Hamiltonian, is in good agreement with experiment, thus validating the potential energy surfaces and their couplings.

Speeding up spin-component-scaled third-order pertubation theory with the chain of spheres approximation: the COSX-SCS-MP3 method

The ‘chain of spheres’ approximation, developed earlier for the efficient evaluation of the self-consistent field exchange term, is introduced here into the evaluation of the external exchange term of higher order correlation methods. Its performance is studied in the specific case of the spin-component-scaled third-order Møller––Plesset perturbation (SCS-MP3) theory. The results indicate that the approximation performs excellently in terms of both computer time and achievable accuracy. Significant speedups over a conventional method are obtained for larger systems and basis sets. Owing to this development, SCS-MP3 calculations on molecules of the size of penicillin (42 atoms) with a polarised triple-zeta basis set can be performed in ∼3 hours using 16 cores of an Intel Xeon E7-8837 processor with a 2.67 GHz clock speed, which represents a speedup by a factor of 8–9 compared to the previously most efficient algorithm. Thus, the increased accuracy offered by SCS-MP3 can now be explored for at least medium-sized molecules.

Assessment of New Meta and Hybrid Meta Density Functionals for Predicting the Geometry and Binding Energy of a Challenging System: The Dimer of H2S and Benzene

Noncovalent interactions of a hydrogen bond donor with an aromatic pi system present a challenge for density functional theory, and most density functionals do not perform well for this kind of interaction. Here we test seven recent density functionals from our research group, along with the popular B3LYP functional, for the dimer of H 2S with benzene. The functionals considered include the four new meta and hybrid meta density functionals of the M06 suite, three slightly older hybrid meta functionals, and the B3LYP hybrid functional, and they were tested for their abilities to predict the dissociation energies of three conformations of the H 2S-benzene dimer and to reproduce the key geometric parameters of the equilibrium conformation of this dimer. All of the functionals tested except B3LYP correctly predict which of the three conformations of the dimer is the most stable. The functionals that are best able to reproduce the geometry of the equilibrium conformation of the dimer with a polarized triple-zeta basis set are M06-L, PWB6K, and MPWB1K, each having a mean unsigned relative error across the two experimentally verifiable geometric parameters of only 8%. The success of M06-L is very encouraging because it is a local functional, which reduces the cost for large simulations. The M05-2X functional yields the most accurate binding energy of a conformation of the dimer for which a binding energy calculated at the CCSD(T) level of theory is available; M05-2X gives a binding energy for the system with a difference of merely 0.02 kcal/mol from that obtained by the CCSD(T) calculation. The M06 functional performs well in both categories by yielding a good representation of the geometry of the equilibrium structure and by giving a binding energy that is only 0.19 kcal/mol different from that calculated by CCSD(T). We conclude that the new generation of density functionals should be useful for a variety of problems in biochemistry and materials where aromatic functional groups can serve as hydrogen bond acceptors.

DNA Base Trimers: Empirical and Quantum Chemical Ab Initio Calculations versus Experiment in Vacuo

A complete scan of the potential-energy surfaces for selected DNA base trimers has been performed by a molecular dynamics/quenching technique using the force field of Cornell et al. implemented in the AMBER7 program. The resulting most stable/populated structures were then reoptimized at a correlated ab initio level by employing resolution of the identity, Møller-Plesset second-order perturbation theory (RI-MP2). A systematic study of these trimers at such a complete level of electronic structure theory is presented for the first time. We show that prior experimental and theoretical interpretations were incorrect in assuming that the most stable structures of the methylated trimers corresponded to planar systems characterized by cyclic intermolecular hydrogen bonding. We found that stacked structures of two bases with the third base in a T-shape arrangement are the global minima in all of the methylated systems: they are more stable than the cyclic planar structures by about 10 kcal mol(-1). The different behaviors of nonmethylated and methylated trimers is also discussed. The high-level geometries and interaction energies computed for the trimers serve also as a reference for the testing of recently developed density functional theory (DFT) functionals with respect to their ability to correctly describe the balance between the electrostatic and dispersion contributions that bind these trimers together. The recently reported M052X functional with a polarized triple-zeta basis set predicts 11 uracil trimer interaction energies with a root-mean-square error of 2.3 kcal mol(-1) relative to highly correlated ab initio theoretical calculations.

Theoretical study of UX 6 and UO 2X 2 (X=F, Cl, Br, I)

In the present study the structural, vibrational and bonding properties of the uranium(VI) halide and oxyhalide molecules UX 6 and UO 2X 2 (X=F, Cl, Br, I) have been investigated by quantum chemical calculations. The title molecular properties have been computed using quasi-relativistic density functional theory in conjunction with a polarised triple-zeta basis set. The bonding interactions between the fragments U⋯X 6 and UO 2⋯X 2 have been investigated using the extended transition state energy partitioning method. Our quantitative analysis revealed an increase of the electrostatic interactions in UO 2X 2 with respect to UX 6 as a result of the oxygen substitution. Analogously to previous results on uranyl, the bonding Kohn–Sham orbitals of UO 2X 2 showed a clear triple-bond character of the formally UO double bonds. The population analysis results are in agreement with a predominant role of the uranium 5f orbitals in the orbital interactions, whereas a slight strengthening of the 6d contributions toward the heavier halides.

DFT calculation of core-electron binding energies

A total of 59 core-electron binding energies (CEBEs) were studied with the Amsterdam Density Functional Program (ADF) program and compared with the observed values. The results indicate that a polarized triple-zeta basis set of Slater-type orbitals is adequate for routine assessment of the performance of each method of computation. With such a basis set, seven density functionals were tested. In addition, the performance of 21 energy density functionals were computed from the density calculated with the statistical average of orbital potentials (SAOP). Among all the choices tested, the best density functional for core-electron binding energies of C to F turns out to be the combination of Perdew-Wang (1986) functional for exchange and the Perdew-Wang (1991) functional for correlation, confirming earlier studies based on contracted Gaussian-type orbitals. For this best functional, five Slater-type orbital basis sets were examined, ranging from polarized double-zeta quality to the largest set available in the ADF package. For the best functional with the best basis set, the average absolute deviation (AAD) of the calculated value from experiment is only 0.16 eV.

Theoretical study of the He–HCN, Ne–HCN, Ar–HCN, and Kr–HCN complexes

The two-dimensional potential energy surfaces for the He–HCN, Ne–HCN, Ar–HCN, and Kr–HCN complexes are presented. Calculations have been performed using single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)] and the augmented correlation-consistent polarized triple-zeta basis set (aug-cc-pVTZ) with an additional (3s3p2d2f1g) set of bond functions. The potentials have been used to find the vibration–rotation energies of the four complexes and their deuterated analogs. The frequencies of rotational or rovibrational transitions found for He–HCN and Ar–HCN are in very good agreement with the experimental results. Good agreement is also obtained with the experimental rotational transition frequencies for Kr–HCN. For Ne–HCN, on the other hand, the agreement with the experimental data is not as good, but can be improved by using larger basis sets.

Coupled ab initio potential energy surfaces for the two lowest 2A′ electronic states of the C2H molecule

Realistic two-valued potential energy surfaces for the reaction C(3P) + CH(X2Π) → C2 + H have been constructed from a set of high level ab initio data describing the first two 2A′ electronic states of the C2H system. These states have linear equilibrium configurations, known as the X 2Σ+ and A2Π states, and are coupled by a conical intersection. They lead to the formation of C2(X1Σ+ g) and C2(a3Πu) considering an adiabatic dissociation process. The ab initio calculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using the ab initio adiabatic energies and the matrix elements of the dipole moment, a 2 × 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Σ/Π conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with the ab initio ones, and good qualitative agreement is observed.

Global analytical representations of the three lowest potential energy surfaces of C2H, and rate constant calculations for the C(3P)+CH(2Π) reaction

Potential energy surfaces for the first three electronic states of the reaction C(3P)+CH(X 2Π)→C2+H have been constructed from a new set of high level ab initio data which are of the multireference configuration interaction variety and were carried out using a polarised triple-zeta basis set. These are the X 2Σ+ and the A 2Π states, and lead to the formation of C2(X 1Σg+) and C2(a 3Πu) considering an adiabatic dissociation process. Each adiabatic potential is expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree–Fock approximate correlation energy model (EHFACE). Moreover, a quasiclassical trajectory study of the title reaction has been performed for each of the three potential energy surfaces, yielding the corresponding rate constants.

Experimental and ab initio equilibrium structure of trans-diazene HNNH

The experimental r z and r e structures of trans-diazene have been calculated using the experimental ground state rotational constants of N 2D 2, N 2D 2 and N 2DH and a newly determined harmonic force field. For comparison the structure has been calculated ab initio at the MP2, CCSD and CCSD(T) levels using a polarized triple-zeta basis set. The average geometry is determined to be: r z ( NH) = 1.041(1) A ̊ , r z ( NN) = 1.252(1) A ̊ and ∠(HNN) = 106.3(1)° and the equilibrium one: r e( NH) = 1.029(1) A ̊ , r e ( NN) = 1.247(1) A ̊ and ∠(HNN) = 106.3(1)°. The Coriolis interacting bands in HNNH. DNND and HNND were reanalysed using the Coriolis constants calculated from the force field.

Equilibrium structure of PH 2Br

The structure of monobromophosphane has been calculated ab initio at the MP2 and CCSD(T) levels using a polarized triple-zeta basis set. For comparison, a relativistic compact effective core potential has also been used for Br. Equilibrium and ground state rotational constants as well as centrifugal distortion constants have been predicted for both isotopic species PH 2 79Br and PH 2 81Br. The r z and r e structures have bee determined by combining the experimental rotational constants and the ab initio geometry and force field.