Electron Correlation Contributions Research Articles

Basis-set extrapolation techniques for the accurate calculation of molecular equilibrium geometries using coupled-cluster theory

To reduce remaining basis-set errors in the determination of molecular equilibrium geometries, a basis-set extrapolation (BSE) scheme is suggested for the forces used in geometry optimizations. The proposed BSE scheme is based on separating the Hartree-Fock and electron-correlation contributions and uses expressions obtained by straightforward differentiation of well established extrapolation formulas for energies when using basis sets from Dunning's hierarchy of correlation-consistent basis sets. Comparison with reference data obtained at the R12 coupled-cluster level [CCSD(T)-R12] demonstrates that BSE significantly accelerates the convergence to the basis-set limit, thus leading to improvements comparable to or even better than those obtained by increasing the cardinal number in the used basis set by one. However, BSE alone is insufficient to improve agreement with experiment, even after additional consideration of inner-shell correlation and quadruple-excitation effects (mean error and standard deviation with extrapolation are -0.014 and 0.047 pm in comparison with mean error and standard deviation of -0.002 and 0.036 pm without extrapolation). Improvement is obtained only when other contributions of similar magnitude as the BSE contributions (e.g., pentuple-excitation effects and relativistic effects) are also considered. A rather large discrepancy (of the order of a few tenths of a picometer) is observed for the F(2) molecule indicating an enhanced basis-set requirement for the various contributions in this case.

On the role of high excitations in the intermolecular potential of H2–CO

Basis set limit explicitly correlated CCSD(T)-R12 calibration values have been calculated for the interaction potential of the H2–CO complex in the vicinity of the two essential van der Waals minima. Correlation consistent cc-p(C)VXZ and aug-cc-p(C)VXZ (X = D,T) basis sets and midbond functions have been further used to examine the importance of higher order effects arising beyond CCSD(T). It is shown that both the missing contributions from the triple excitations and from the quadruple excitations affect the final potential appreciably and anisotropically. Their total value in the vicinity of the global minimum amounts to about 3 cm−1, whereas only about 1 cm−1 at the secondary minimum, making the minima deeper in both cases. Other corrections are much smaller. The contribution of the core electron correlation is lower by one order of magnitude. The diagonal Born–Oppenheimer and relativistic corrections are also very small and, due to partial mutual cancellation effects, their total contribution affects the interaction potential below the 0.1 cm−1 level.

Characteristics of Antiaromatic Ring π Multi-Hydrogen Bonds in (H2O)n−C4H4(n= 1, 2) Complexes

By counterpoise-corrected optimization method, the six antiaromatic ring pi multi-hydrogen bond structures with diversiform shapes for (H2O)n-C4H4 (n = 1,2) have been obtained at the MP2/aug-cc-pVDZ level. At the CCSD(T)/aug-cc-pVDZ level, the interaction energy obtained mainly depends on the numbers of H2O and fold numbers of the pi multi-hydrogen bond. The interaction energy order is -2.342 (1a with pi mono-hydrogen) < -2.777 (1b with pi bi-hydrogen) << -4.683 (2a with pi bi-hydrogen) < -4.734 (2b with pi tri-hydrogen) < -4.782 (2c with pi tri-hydrogen) < -5.009 kcal/mol (2d with pi tetra-hydrogen bond). Strangely, why is the interaction energy of the pi bi-hydrogen bond in 1b close to that of the pi mono-hydrogen bond in 1a (their difference is only 15.7%)? The reason is that a pi-type H-bond (as an accompanying interaction) between two lone pairs of the O-atom and a near pair of H-atoms of C4H4 exists shoulder by shoulder in structures 1a, 2a, 2b, and 2c and contributes to the interaction energy. Another accompanying interaction, a repulsive interaction between the pi H-bond (using the H-atom(s) of H2O) and the near pair of H-atoms of C4H4, is also found. For the structures and interaction energies, the pi-type H-bond produces four effects: bending the strong pi H-bond, attracting the pair of H-atoms of C4H4 so that they deviate from the C4 ring plane, showing the interaction energy contribution, and bringing the larger electron correlation contribution. The repulsive interaction also produces four effects: pushing the pair of H-atoms of C4H4 so that they deviate from its ring plane, elongating the distance of the pi H-bond, promoting the formation of pi-type H-bond, and slightly influencing the interaction energy. In the present paper, one C=C bond with two H2O (over and below the ring plane) forms a pi H-bond link in two ways: a strong-weak pi H-bond link and a strong-strong pi H-bond link. The stability contribution of the former is more favorable than the latter. One H2O forms a pi H-bond with C4H4 in two ways. One strong pi H-bond part (over or below the ring plane) always is accompanied by another H-bond part. The accompanying part is either a weak pi H-bond or pi-type H-bond.

Nature of Base Stacking: Reference Quantum‐Chemical Stacking Energies in Ten Unique B‐DNA Base‐Pair Steps

Base-stacking energies in ten unique B-DNA base-pair steps and some other arrangements were evaluated by the second-order Møller-Plesset (MP2) method, complete basis set (CBS) extrapolation, and correction for triple (T) electron-correlation contributions. The CBS(T) calculations were compared with decade-old MP2/6-31G*(0.25) reference data and AMBER force field. The new calculations show modest increases in stacking stabilization compared to the MP2/6-31G*(0.25) data and surprisingly large sequence-dependent variation of stacking energies. The absolute force-field values are in better agreement with the new reference data, while relative discrepancies between quantum-chemical (QM) and force-field values increase modestly. Nevertheless, the force field provides good qualitative description of stacking, and there is no need to introduce additional pair-additive electrostatic terms, such as distributed multipoles or out-of-plane charges. There is a rather surprising difference of about 0.1 A between the vertical separation of base pairs predicted by quantum chemistry and derived from crystal structures. Evaluations of different local arrangements of the 5'-CG-3' step indicate a sensitivity of the relative stacking energies to the level of calculation. Thus, describing quantitative relations between local DNA geometrical variations and stacking may be more complicated than usually assumed. The reference calculations are complemented by continuum-solvent assessment of solvent-screening effects.

Development of Many−Body Polarizable Force Fields for Li-Battery Components: 1. Ether, Alkane, and Carbonate-Based Solvents

Classical many-body polarizable force fields were developed for n-alkanes, perflouroalkanes, polyethers, ketones, and linear and cyclic carbonates on the basis of quantum chemistry dimer energies of model compounds and empirical thermodynamic liquid-state properties. The dependence of the electron correlation contribution to the dimer binding energy on basis-set size and level of theory was investigated as a function of molecular separation for a number of alkane, ether, and ketone dimers. Molecular dynamics (MD) simulations of the force fields accurately predicted structural, dynamic, and transport properties of liquids and unentangled polymer melts. On average, gas-phase dimer binding energies predicted with the force field were between those from MP2/aug-cc-pvDz and MP2/aug-cc-pvTz quantum chemistry calculations.

Cyclobutadiene complexed with Li atom and the effects of substituting by F atom in LiC 4H 4

The 12 structures of the LiC 4H 4− n F n ( n=0–4) complexes with all real frequencies are got at the MP2/6-311++G** level. These complexes are all charge-separated species Li +(C 4H 4− n F n ) − with large interaction energies (−2.13 to −5.34 eV) at the CCSD(T)/6-311++G** level. Different from benzene ring which acts as an electron donor in Li·C 6H 6, the cyclic C 4H 4− n F n ( n=0–4) are electron acceptors. Interestingly, in the LiC 4H 4 complex, the subunit C 4H 4 presents an out-of-plane ‘roof’ structure ( C 2 v ), which differs from the planar rhombus or rectangular structure of C 4 H 4 − , and also differs from the square structure ( D 4 h ) of C 4 H 4 2 − in the LiC 4H 4Li complex. The complex in LiC 4H 4− n F n ( n=1–4) has low symmetry ( C s or C 1) due to F substituent(s). The LiC 4H 4− n F n complexes with F(2) substituent exhibit different structural characteristics from those without F(2) substituent. Thus the 11 F-substituted complexes plus LiC 4H 4 are clearly divided into A (without F(2)) and B (with F(2)) two groups. The electron correlation contributions are obviously different between the complexes in group A and those in group B for interaction energies and vertical ionization energies (VIE).

Molecular equilibrium geometries based on coupled-cluster calculations including quadruple excitations*

Using analytic gradient techniques and an additivity scheme for the various electron correlation contributions, i.e. core-correlation, contribution due to full treatment of triple excitations and contributions due to quadruple excitations calculated with different basis sets, the accuracy of computed geometrical parameters are analysed in comparison with experiment. For a test set of 12 closed-shell and 5 open-shell molecules, it is found that inclusion of quadruple excitations is essential to reach agreement with experiment. The mean error of 0.002 pm and the standard deviation of 0.040 pm of the present CCSD(T)/cc-pV6Z + core(CCSD(T)/cc-pCVQZ) + T/cc-pVTZ + Q/cc-pVDZ results for the closed-shell systems underline the importance of quadruple excitations, in particular, as corresponding calculations without quadruple excitations exhibit significantly larger error. Quadruples contributions for multiply bonded systems as well as the challenging F2 molecule are as large as 0.1 to 0.3 pm, while for single bonds the effects are typically significantly smaller than 0.1 pm.

Ab Initio Calculations and Franck–Condon Simulation of the Absorption Spectra of GeCl2 Including Anharmonicity

Geometrical parameters, vibrational frequencies and relative electronic energies of the X1A1, ã3B1 and A1B1 states of GeCl2 have been calculated at the CCSD(T) and/or CASSCF/MRCI level with basis sets of up to aug-cc-pV5Z quality. Core electron correlation and relativistic contributions were also investigated. RCCSD(T)/ aug-cc-pVQZ potential energy functions (PEFs) of the X1A1 and ã3B, states, and a CASSCF/MRCl/aug-cc-pVQZ PEF of the A1B1 state of GeCl2 are reported. Anharmonic vibrational wavefunctions of these electronic states of GeCl2, obtained variationally using the computed PEFs, are employed to calculate the Franck-Condon factors (FCFs) of the ã-X and A-X transitions of GeCl2. Simulated absorption spectra of these transitions based on the computed FCFs are compared with the corresponding experimental laser-induced fluorescence (LIF) spectra of Karolczak et al. [J. Chem. Phys. 1993, 98, 60-70]. Excellent agreement is obtained between the simulated absorption spectrum and observed LIF spectrum of the ã-X transition of GeCl2, which confirms the molecular carrier, the electronic states involved and the vibrational assignments of the LIF spectrum. However, comparison between the simulated absorption spectrum and experimental LIF spectrum of the A-X transition of GeCl2 leads to a revision of vibrational assignments of the LIF spectrum and suggests that the X1A1 state of GeCl2 was prepared in the experimental work, with a non-Boltzmann vibrational population distribution. The X(0,0,1) level is populated over 4000 times more than expected from a Boltzmann distribution at 60 K, which is appropriate for the relative population of the other low-lying vibrational levels, such as the X(1,0,0) and X(0,1,0) levels.

Histidine-Aromatic Interactions in Proteins and Protein-Ligand Complexes: Quantum Chemical Study of X-ray and Model Structures.

His-aromatic complexes, with the His located above the aromatic plane, are stabilized by π-π, δ(+)-π and/or cation-π interactions according to whether the His is neutral or protonated and the partners are in stacked or T-shape conformations. Here we attempt to probe the relative strength of these interactions as a function of the geometry and protonation state, in gas phase, in water and protein-like environments (acetone, THF and CCl4), by means of quantum chemistry calculations performed up to second order of the Møller-Plesset pertubation theory. Two sets of conformations are considered for that purpose. The first set contains 89 interactions between His and Phe, Tyr, Trp, or Ade, observed in X-ray structures of proteins and protein-ligand complexes. The second set contains model structures obtained by moving an imidazolium/imidazole moiety above a benzene ring or an adenine moiety. We found that the protonated complexes are much more stable than the neutral ones in gas phase. This higher stability is due to the electrostatic contributions, the electron correlation contributions being equally important in the two forms. Thus, π-π and δ(+)-π interactions present essentially favorable electron correlation energy terms, whereas cation-π interactions feature in addition favorable electrostatic energies. The protonated complexes remain more stable than the neutral ones in protein-like environments, but the difference is drastically reduced. Furthermore, the T-shape conformation is undoubtedly more favorable than the stacked one in gas phase. This advantage decreases in the solvents, and the stacked conformation becomes even slightly more favorable in water. The frequent occurrence of His-aromatic interactions in catalytic sites, at protein-DNA or protein-ligand interfaces and in 3D domain swapping proteins emphasize their importance in biological processes.

Study of π Halogen Bonds in Complexes C2H4-nFn−ClF (n = 0−2)

The pi-halogen bond may be considered, in a broad sense, essentially a pi-hydrogen bond. Using the counterpoise-corrected potential energy surface method (interaction energy optimization), the stationary structures of the C2H(4-n)Fn-ClF (n = 0-2) complexes with all real frequencies have been obtained at the MP2/aug-cc-pVDZ level. For C2H(4-n)Fn-ClF (n = 0-2), the pi-halogen bond has a long distance and is elongated by the F substituent effect. The pi-halogen bond length order is 2.661 A for C2H4-ClF < 2.745 A for C2H3F-ClF < 2.766 A for g-C2H2F2-ClF < 2.8076 A for trans-C2H2F2-ClF < 2.8079 A for cis-C2H2F2-ClF. For three complexes C2H3F-ClF, g-C2H2F2-ClF, and cis-C2H2F2-ClF, the pi-halogen bonds are further shifted and sloped by the F substituent effect. The F substituent effect reduces also the interaction energy of the pi-halogen bond. The interaction energies are respectively -3.7 for C2H4-ClF, -2.8 for C2H3F-ClF, -2.3 for g-C2H2F2-ClF, -1.9 for cis-C2H2F2-ClF, and -1.8 kcal/mol for trans-C2H2F2-ClF, at the CCSD(T)/aug-cc-pVDZ level. The electron correlation contribution of the interaction energy is large for C2H(4-n)Fn-ClF (n = 0-2), which shows that the stabilities of the pi-halogen bond systems results primarily from the dispersion interaction. In the double F substituent systems, the interaction energy of the pi-halogen bond structure with a longer interaction distance is larger than that of the corresponding pi-hydrogen bond structure with a shorter interaction distance. This may be because there are the large electron correlation contributions of the interaction energy, and a secondary interaction between lone pairs of Cl atom and some atoms (H, C) with positive charges in the pi-halogen bond structure.

Relativistic second-order many-body and density-functional theory for the parity-violation contribution to theC‐Fstretching mode in CHFClBr

Relativistic four-component electronic structure theory using both wave-function (Dirac-Coulomb-Hartree-Fock and second-order many-body perturbation-theory) and density-functional based methods (local density, hybrid, and generalized gradient approximations) is applied to discuss the current status on the accuracy of parity-violation calculations for molecules. As a test case we choose the C-F stretching mode of CHFClBr, which is currently being investigated by molecular-beam spectroscopy. We show that electron correlation effects are important and cannot be neglected anymore for the parity nonconservation contribution to the total electronic energy. However, electron correlation contributions to parity violation in vibrational transitions of the C-F stretching mode are less important. The density functionals tested give somewhat different results, but the Becke three-parameter Lee-Yang-Parr functional agrees quite well with the second-order many-body perturbation-theory values. The calculations suggest that electron correlation effects have to be considered for future investigations in parity-violation effects in electronic transitions. The performance of density-functional based methods for this property needs further statistics.

NLO response of polymethineimine and polymethineimine/polyacetylene conformers: Assessment of electron correlation effects

AbstractWe have studied, at the second‐order Møller–Plesset level of approximation with the 6‐31G(d) atomic basis set, the electron correlation effects on the first hyperpolarizability of increasingly long polymethineimine oligomers and alternating polymethineimine/polyacetylene copolymers showing different conformations. Both the direct (i.e., electronic) and indirect (i.e., geometric) electron correlation contributions are large and should consequently be accounted for, even for qualitative purposes. The electron correlation effects are both conformation and chain length dependent. Long alternating polymethineimine/polyacetylene copolymers present a very large first hyperpolarizabilities, especially in the all‐trans conformation. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Relativistic Gaussian basis sets obtained with a polynomial version of the generator coordinate Dirac–Fock method: Ionization energies of some closed‐shell atomic systems

AbstractThe purpose of this work is to analyze the relativistic Gaussian basis sets obtained with a recently developed polynomial version of the generator coordinate Dirac–Fock method. The atomic systems chosen to evaluate the basis sets are some alkaline earth and noble gas atoms: He, Be, Ne, Mg, Ar, and Ca. Hartree–Fock, relativistic, and electronic correlation contributions to the atomic total energy of the neutral and ionized atomic species were estimated using these relativistic Gaussian basis sets. The basis sets developed were able to attain energies that deviate at most by 0.47 and 0.65 mHartree, respectively, from numerical Hartree–Fock and Dirac–Fock–Coulomb results and are also competitive with fully optimized relativistic Gaussian basis sets of similar sizes. The electronic correlation energy contributions were obtained after two nonrelativistic calculations: a Hartree–Fock and a full quadratic configuration interaction, including single and double substitutions. The uncontracted basis sets obtained here along with polarization functions chosen to have equal exponents to selected functions already present in the original set were adequate in describing not only the neutral but also the highly ionized atomic species. The comparison of some calculated shell ionization energies with experimental results presented a maximum deviation of 3% (for a total of 15 ionization values ranging from 18 to 10,599 eV), reinforcing the quality of the basis sets developed in this work. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

High excitations in coupled-cluster series: vibrational energy levels of ammonia

The ammonia molecule containing large amplitude inversion motion is a revealing system in examining high-order correlation effects on potential energy surfaces. Correlation contributions to the equilibrium and saddle point geometries, inversion barrier height and vibrational energy levels, including inversion splittings, have been investigated. A six-dimensional Taylor-type series expansion of the Born–Oppenheimer potential energy surface, which is scaled to different levels of theory, is used to determine vibrational energy levels and inversion splittings variationally. The electronic energies are calculated by coupled-cluster methods, combining explicitly correlated R12 theory (which includes the interelectronic coordinate in the electronic wave function) with a conventional approach including excitations up to the pentuple level. Finally, the electronic correlation contribution is scaled to the full configuration interaction limit. Corrections due to relativistic and non-Born–Oppenheimer effects are also included. Special emphasis is put on the convergence of the high-order contributions with respect to the size of the atomic basis set. To achieve an accuracy of 1 cm−1, it is essential to be at the basis set limit, include all the subtle effects and also include highly excited configurations—even up to the pentuple level in the coupled-cluster expansion.

Convergence of theab initiomany-body expansion for the cohesive energy of solid mercury

A many-body expansion for mercury clusters of the form $E={\ensuremath{\sum}}_{i}{ϵ}_{i}+{\ensuremath{\sum}}_{i&lt;j}\ensuremath{\Delta}{ϵ}_{ij}+{\ensuremath{\sum}}_{i&lt;j&lt;k}\ensuremath{\Delta}{ϵ}_{ijk}+\dots{},$ does not converge smoothly with increasing cluster size towards the solid state. Even for smaller cluster sizes (up to $n=6$), where van der Waals forces still dominate, one observes bad convergence behavior. For solid mercury the convergence of the many-body expansion can dramatically be improved by an incremental procedure within an embedded cluster approach. Here one adds the coupled cluster many-body electron correlation contributions of the embedded cluster to the bulk HF energy. In this way we obtain a cohesive energy (not corrected for zero-point vibration) of $0.79\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in very good agreement with the experimental value of $0.79\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.

Additivity of Cation−π Interactions: An ab Initio Computational Study on π−Cation−π Sandwich Complexes

The π−cation−π interaction between a cation or a cationic group and several aromatic residues, although rather prevalent in biological systems, has not been studied theoretically. The ab initio MP2 calculations were carried out on the systems composed of TMA with two aromatic rings, viz. benzene, pyrrole, or indole, to explore how a cation or a cationic group interacts simultaneously with two aromatic residues in proteins or nucleic acids. The calculated results on π−TMA−π complexes revealed additivities of both the geometries and the binding energies relative to cation−π complexes. The preferred structure of such a complex can be constructed by superimposing the corresponding TMA−π complexes via the cation. The binding energies of the π−TMA−π sandwiches are the sums of the two corresponding TMA−π systems. The contribution of electron correlation to the overall binding energy is estimated to be at least 50%, with dispersion serving as the main component of the electron correlation interaction. Similar to geometrical and energetic additivity, the additivities in BSSE and ΔZPE were also found. Therefore, our finding provides a convenient and effective way to construct π−TMA−π sandwiches and to estimate their binding energies. Morokuma decomposition analysis on the binding energy indicated that the electrostatic, charge transfer, and polarity interactions drive the binding of TMA with aromatics, whereas the exchange repulsion and high order coupling always obstruct the TMA approaching aromatics. Charge-transfer happens to some extent during the complexation of TMA with aromatics, and the transferred NPA atomic charges and charge-transfer energies are almost same in different complexes of TMA−π or π−TMA−π. The interaction between the 2 aromatics in the sandwich π−TMA−π complexes is negligible because of their long interaction distances. All this information should be helpful in studying such interactions in biological systems.

Geometry, dipole moment, polarizability and first hyperpolarizability of polymethineimine: an assessment of electron correlation contributions.

We have investigated the geometries as well as the longitudinal dipole moment (micro), polarizability (alpha), and first hyperpolarizability (beta) of polymethineimine oligomers using different approaches [Hartree-Fock (HF), second-order Møller-Plesset (MP2), and hybrid density functional theory (DFT) methods (B3LYP and PBE0)] for evaluating the geometries and the nonlinear optical properties. It turns out that (i) HF and the selected DFT methods provide the incorrect sign for beta of short and medium size oligomers. (ii) The B3LYP and PBE0 electron correlation correction are too small for micro, too large for alpha, and for some oligomer lengths, they are in the wrong direction for beta. (iii) On the contrary to polyacetylene, the hybrid-DFT geometries are in poor agreement with MP2 geometries; the former showing much smaller bond length alternations.

The Xe shielding surfaces for Xe interacting with linear molecules and spherical tops.

The 129Xe nuclear magnetic resonance spectrum of xenon in gas mixtures of Xe with other molecules provides a test of the ab initio surfaces for the intermolecular shielding of Xe in the presence of the other molecule. We examine the electron correlation contributions to the Xe-CO2, Xe-N2, Xe-CO, Xe-CH4, and Xe-CF4 shielding surfaces and test the calculations against the experimental temperature dependence of the density coefficients of the Xe chemical shift in the gas mixtures at infinite dilution in Xe. Comparisons with the gas phase data permit the refinement of site-site potential functions for Xe-N2, Xe-CO, and Xe-CF4 especially for atom-Xe distances in the range 3.5-6 A. With the atom-atom shielding surfaces and potential parameters obtained in the present work, construction of shielding surfaces and potentials for applications such as molecular dynamics averaging of Xe chemical shifts in liquid solvents containing CH3, CH2, CF3, and CF2 groups is possible.

Confined quantum systems: dipole polarizability of the two-electron quantum dot,the hydrogen negative ion and the helium atom

The dipole polarizabilities of the electronic ground state of three two-electronquantum systems, namely the two-electron quantum dot, the hydrogen negativeion and the helium atom, confined by an anisotropic harmonic oscillatorpotential have been studied for different confinement strengths ωby using the quantum chemical configuration interaction (CI) methodemploying a Cartesian anisotropic Gaussian basis set supplemented by aquantum-chemical standard Cartesian Gaussian basis set, respectively,and a full CI wavefunction. The dipole polarizability of the harmonicoscillator quantum dot may be calculated analytically. It is proportional tothe number of electrons, inversely proportional to the square of ω andhas no electron correlation contribution. This is valid for all states includingthe excited states. The situation is different for quantum systems witha finite nuclear charge, like the hydrogen negative ion and the heliumatom. In general the dipole polarizability of the two-electron quantumdot decreases strongly with increasing confinement strength ω, thepolarizability of the helium atom varying slowly with increasing ω,while the behaviour of the polarizability of the hydrogen negativeion lies between both. Except for the two-electron quantum doteach component of the dipole polarizability tensor, i.e. αxx, αyy andαzz,depends on the confinement along all axes. The electron correlation contributionto the dipole polarizability of the helium atom and of the hydrogen negativeion behaves differently for the different Cartesian components αzz andαxxof the tensor. The different behaviour observed for different Cartesian componentsmay be rationalized by the reduced dimensionality of the systems.

Bound states of positron with urea and acetone molecules using configuration interaction ab initio molecular orbital approach

Characteristic features of the positron binding structure of the urea and acetone molecules are discussed based on the results obtained by the configuration interaction scheme of quantum chemical molecular orbital calculations. This method takes the positron–electron correlation contribution into account explicitly. We have found that the positron distribution is concentrated behind the carbonyl oxygen atom. The positron affinity of urea is larger than that of acetone, which is consistent with the stronger dipole moment of urea.