An energy decomposition scheme based on the block-localized wave function (BLW) method is proposed. The key of this scheme is the definition and the full optimization of the diabatic state wave function, where the charge transfer among interacting molecules is deactivated. The present energy decomposition (ED), BLW-ED, method is similar to the Morokuma decomposition scheme in definition of the energy terms, but differs in implementation and the computational algorithm. In addition, in the BLW-ED approach, the basis set superposition error is fully taken into account. The application of this scheme to the water dimer and the lithium cation–water clusters reveals that there is minimal charge transfer effect in hydrogen-bonded complexes. At the HF/aug-cc-PVTZ level, the electrostatic, polarization, and charge-transfer effects contribute 65%, 24%, and 11%, respectively, to the total bonding energy (−3.84 kcal/mol) in the water dimer. On the other hand, charge transfer effects are shown to be significant in Lewis acid–base complexes such as H3NSO3 and H3NBH3. In this work, the effect of basis sets used on the energy decomposition analysis is addressed and the results manifest that the present energy decomposition scheme is stable with a modest size of basis functions.

A block-localized wave function method is introduced to evaluate the electronic delocalization effect in molecules. The wave function for the hypothetical and strictly localized structure is constructed based on the assumption that all electrons and primitive basis functions can be divided into several subgroups; each localized molecular orbital is expanded in terms of primitive orbitals belonging to only one subgroup. The molecular orbitals belonging to the same subgroup are constrained to be mutually orthogonal, while those belonging to different subgroups are free to overlap. The final block-localized wave function at the Hartree–Fock level is expressed by a Slater determinant. In this manner, the energy difference between the Hartree–Fock wave function and the block-localized wave function can be generally defined as the electronic delocalization energy. The method is applied to two cases. The first concerns the resonance stabilization in the allyl ions. We find that the vertical resonance energies for the planar cation and anion are −45.7 (or −44.7) and −46.7 (or −48.2) kcal/mol at the HF/6-31G* (or 6-31+G*) level, respectively. Their rotational barriers are decomposed in terms of conjugation, hyperconjugation, steric effect, and pyramidalization. The n→σ* negative hyperconjugation in the staggered allyl anion is very strong and stabilizes the system by as much as −13 kcal/mol. The second concerns the hyperconjugation effect in propene. Our calculations suggest that the theoretical hyperconjugation energy in propene is about −5 kcal/mol, which is close to the experimental estimate (−2.7 kcal/mol) derived from the hydrogenation heats of propene and ethylene. Comparisons between the results based on the present block-localized wave function method and those based on the natural bond orbital method are presented and discussed. The examples demonstrate that the block-localized wave function method can be employed as a useful model to analyze chemical bondings and intuitive concepts.

Orotidine 5 -monophosphate decarboxylase catalyzes the conversion of orotidine 5 -monophosphate to uridine 5 -monophosphate, the last step in biosynthesis of pyrimidine nucleotides. As part of a Structural Genomics Initiative, the crystal structures of the ligand-free and the6-azauridine 5 -monophosphate-complexed forms have been determined at 1.8 and 1.5 Å, respectively. The protein assumes a TIM-barrel fold with one side of the barrel closed off and the other side binding the inhibitor. A unique array of alternating charges (Lys-Asp-Lys-Asp) in the active site prompted us to apply quantum mechanical and molecular dynamics calculations to analyze the relative contributions of ground state destabilization and transition state stabilization to catalysis. The remarkable catalytic power of orotidine 5 -monophosphate decarboxylase is almost exclusively achieved via destabilization of the reactive part of the substrate, which is compensated for by strong binding of the phosphate and ribose groups. The computational results are consistent with a catalytic mechanism that is characterized by Jencks's Circe effect.O rotidine 5Ј-monophosphate decarboxylase (ODCase) (EC 4.1.1.23) formally catalyzes the exchange of CO 2 for a proton at the C 6 position to form uridine 5Ј-monophosphate (UMP) (1). The intermediate implied by this description consists of a C 6 -carbanion, the conjugate base of the UMP carbon acid. The ODCase reaction is unique in biological decarboxylation reactions in that the carbanion intermediate is not stabilized by conjugation interactions and, thus, the reaction rate is exceptionally slow in aqueous solution (2). The remarkable catalytic power of ODCase, which accelerates the reaction by 17 orders of magnitude over the aqueous process, has fascinated chemists and biochemists alike, leading to a number of proposals of mechanisms with novel features (3-7). However, as more results accumulated for this class of enzymes, possibilities for the mechanism became increasingly limited as cofactors and catalytic groups continued to be excluded from consideration (8-10). The high-resolution x-ray structure of ODCase from Methanobacterium thermoautotrophicum reveals that the mechanism is almost fully characterized by the formal description, along with electrostatic features of the enzyme's active site that provide selective destabilization of the orotidine group. In what follows, we report the results from a joint experimental and theoretical investigation, providing a mechanism that involves significant ground state destabilization effects in enzyme catalysis (11).The key to ODCase's catalytic power is its ability to utilize a phenomenon, which we classify as electrostatic stress [following Fersht's description of ''stress'' in catalysis (12)]. Although binding of the orotidine 5Ј-monophosphate (OMP) results in significant stabilizing interactions with the phosphate and ribose in the active site as revealed by the x-ray structural analysis, electrostatic interactions between the orotate group and ODCase is strongl...

A multistate density functional theory in the framework of the valence bond model is described. The method is based on a block-localized density functional theory (BLDFT) for the construction of valence-bond-like diabatic electronic states and is suitable for the study of electron transfer reactions and for the representation of reactive potential energy surfaces. The method is equivalent to a valence bond theory with the treatment of the localized configurations by using density functional theory (VBDFT). In VBDFT, the electron densities and energies of the valence bond states are determined by BLDFT. A functional estimate of the off-diagonal matrix elements of the VB Hamiltonian is proposed, making use of the overlap integral between Kohn-Sham determinants and the exchangecorrelation functional for the ground state substituted with the transition (exchange) density. In addition, we describe an approximate approach, in which the off-diagonal matrix element is computed by wave function theory using block-localized Kohn-Sham orbitals. The key feature is that the electron density of the adiabatic ground state is not directly computed nor used to obtain the ground-state energy; the energy is determined by diagonalization of the multistate valence bond Hamiltonian. This represents a departure from the standard single-determinant Kohn-Sham density functional theory. The multistate VBDFT method is illustrated by the bond dissociation of and a set of three nucleophilic substitution reactions in the DBH24 database. In the dissociation of , the VBDFT method yields the correct asymptotic behavior as the two protons stretch to infinity, whereas approximate functionals fail badly. For the S N 2 nucleophilic substitution reactions, the hybrid functional B3LYP severely underestimates the barrier heights, while the approximate two-state VBDFT method overcomes the self-interaction error, and overestimates the barrier heights. Inclusion of the ionic state in a three-state model, VBDFT(3), significantly improves the computed barrier heights, which are found to be in accord with accurate results. The BLDFT method is a versatile theory that can be used to analyze conventional DFT results to gain insight into chemical bonding properties, and it is illustrated by examining the intricate energy contributions to the ion-dipole complex stabilization.

The block-localized wavefunction (BLW) approach is an ab initio valence bond (VB) method incorporating the efficiency of molecular orbital (MO) theory. It can generate the wavefunction for a resonance structure or diabatic state self-consistently by partitioning the overall electrons and primitive orbitals into several subgroups and expanding each block-localized molecular orbital in only one subspace. Although block-localized molecular orbitals in the same subspace are constrained to be orthogonal (a feature of MO theory), orbitals between different subspaces are generally nonorthogonal (a feature of VB theory). The BLW method is particularly useful in the quantification of the electron delocalization (resonance) effect within a molecule and the charge-transfer effect between molecules. In this paper, we extend the BLW method to the density functional theory (DFT) level and implement the BLW-DFT method to the quantum mechanical software GAMESS. Test applications to the pi conjugation in the planar allyl radical and ions with the basis sets of 6-31G(d), 6-31+G(d), 6-311+G(d,p), and cc-pVTZ show that the basis set dependency is insignificant. In addition, the BLW-DFT method can also be used to elucidate the nature of intermolecular interactions. Examples of pi-cation interactions and solute-solvent interactions will be presented and discussed. By expressing each diabatic state with one BLW, the BLW method can be further used to study chemical reactions and electron-transfer processes whose potential energy surfaces are typically described by two or more diabatic states.

A mixed molecular orbital and valence bond (MOVB) method for describing the potential energy surface of reactive systems has been developed and applied to a model proton transfer reaction in aqueous solution. The MOVB method is based on a block-localized wave function (BLW) approach for defining the diabatic electronic states. Then, a configuration interaction Hamiltonian is constructed using these diabatic states as the basis function. It was found that the electronic coupling energy is large with a value of about 30 kcal/mol for the H 3 N-H-NH 3 + system, whereas the predicted activation barrier is only 1.2 kcal/mol using the 3-21G basis set. The MOVB results are found to be in good accord with the corresponding ab initio Hartree-Fock calculations for the proton transfer process. We have also incorporated solvent effects into the MOVB Hamiltonian in the spirit of combined QM/MM calculations, and have modeled the proton transfer between ammonium ion and ammonia in water using Monte Carlo simulations. The potential of mean force was computed via free energy perturbation coupled with umbrella sampling techniques using (1) an energy gap mapping approach, and (2) a geometrical mapping procedure. Solvent effects increase the barrier height by about 2.2 kcal/mol from the MOVB and HF ground state potential energy surface. The present study demonstrated the feasibility of ab initio MOVB method for studying chemical reactions by incorporating explicit solvent effects in the description of the reaction coordinate in combined QM/MM simulations.

An interaction energy decomposition analysis method based on the block-localized wavefunction (BLW-ED) approach is described. The first main feature of the BLW-ED method is that it combines concepts of valence bond and molecular orbital theories such that the intermediate and physically intuitive electron-localized states are variationally optimized by self-consistent field calculations. Furthermore, the block-localization scheme can be used both in wave function theory and in density functional theory, providing a useful tool to gain insights on intermolecular interactions that would otherwise be difficult to obtain using the delocalized Kohn–Sham DFT. These features allow broad applications of the BLW method to energy decomposition (BLW-ED) analysis for intermolecular interactions. In this perspective, we outline theoretical aspects of the BLW-ED method, and illustrate its applications in hydrogen-bonding and π–cation intermolecular interactions as well as metal–carbonyl complexes. Future prospects on the development of a multistate density functional theory (MSDFT) are presented, making use of block-localized electronic states as the basis configurations.

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