Surface energies of Si(0 0 1), (1 1 0), (1 1 1), and (1 1 3) surfaces with different reconstructions are calculated systematically using first-principles total-energy method. In order to quantitatively compare their relative stability, the surface energies of different surface orientations and their respective theoretical bulk atom energies are determined simultaneously by linear fitting slab supercell total energy as a function of the atom number in the slab. Equivalent computational parameters and convergence criteria are used for all calculations. Without considering entropy contribution, the relative stability of these Si surfaces with given reconstructions is shown, in decreasing order, from (1 1 1) to (0 0 1) and (1 1 3) at low temperature, and from (0 0 1), (1 1 3), (1 1 0), to (1 1 1) at high temperature.

Using the first-principles total energy method, we calculate surface energies, surface stresses, and their strain dependence of the Ge-covered Si (001) and (105) surfaces. The surface energy of the Si(105) surface is shown to be higher than that of Si(001), but it can be reduced by the Ge deposition, and becomes almost degenerate with that of the $\mathrm{Ge}∕\mathrm{Si}(001)$ surface for three-monolayer Ge coverage (the wetting layer), leading to the formation of the {105}-faceted Ge hut. The unstrained Si and Ge (105) surfaces are unstable due to the large tensile surface stress originated from the surface reconstruction, but they can be largely stabilized by applying an external compressive strain, such as by the deposition of Ge on Si(105). Our study provides a quantitative understanding of the strain stabilization of $\mathrm{Ge}∕\mathrm{Si}(105)$ surface, and hence the formation of the {105}-faceted Ge huts on Si(001).

We perform extensive first-principles calculations to simulate the topographical atomic-force-microscope image of an adatom on the Si(111)-(7 x 7) surface, demonstrating the feasibility of imaging not only the atoms but also the atomic orbitals. Our comparative study of tip terminations shows that two subatomic features can appear for a single adatom when it is imaged by a Si(001)-type tip having two dangling bonds on its apex, while only one feature would appear if it were imaged by a Si(111)-type tip having one dangling bond on the apex. The key condition for seeing the atomic orbitals is to bring the tip so close to the surface that the angular-dependent force dominates the tip-surface interaction.

Building on the previously developed multistate empirical valence bond model [U. W. Schmitt and G. A. Voth, J. Chem. Phys 111, 9361 (1999)] for the dynamics and energetics of an excess proton in bulk phase water, a second generation model is described. This model is shown to produce similar dynamic and structural properties to the previous model, while allowing for the use of the full hydronium charge. This characteristic of the model is required for its implementation in a host of realistic applications beyond bulk water. An improved state selection algorithm is also presented, resulting in a significantly reduced energy drift during microcanonical molecular dynamics simulations. The unusually high self diffusion constant of an excess proton in water due to the proton hopping (Grotthuss) process is observed in the simulation data and is found to be quantitatively in the same range as the experimental value if a quantum correction is taken into consideration. Importantly, a more complete analysis of proton transport process is also presented.

The development and application of a multistate empirical valence bond (MS-EVB) model for a weak acid dissociation and subsequent proton transport in aqueous solution is described. The weak acid dissociation step is modeled by the inclusion of an additional EVE state describing the case when proton is bound to the acid's conjugate base. The model was parametrized for the imidazolium cation deprotonation. Classical molecular dynamics simulation methodology was used to study both equilibrium and dynamic properties of this system. Free energy profiles of the deprotonation reaction, studied using a novel center of excess charge reaction coordinate, reveal the need to include several solvation shells around the weak acid in order to stabilize the hydronium species formed upon the weak acid deprotonation. The solvent atomic density plots examined at selected points along the proton transfer coordinate display a relatively large reorganization of the solvent around the weak acid molecule, caused by the shift in the weak acid molecule atomic point charges caused by the deprotonation. Finally, since the concentration of the weak acid in the system under study is low, its presence has only mimimal effect on the solvent diffusion and on the transfer dynamics of the excess proton in the water solution after the weak acid dissociation step.

Classical molecular dynamics simulations in conjunction with a multi-state empirical valence bond (MS-EVB) model are used to study proton transport in strong and weak acid aqueous solutions. The strong acid, HCl, is modeled in its ionized state by inserting a chloride counter ion into the protonated water solution. Both equilibrium and dynamical properties differ only slightly from the previously studied isolated excess proton in water. The free-energy profile as a function of the separation between the excess charge and chlorine atom reveal minimal barrier for the anion-excess charge separation. To model the weak acid, protonated imidazole, the MS-EVB model was extended to include the protonated form of the acid in the EVB description, so that the dissociation step can be studied. Free energy profiles for the weak acid deprotonation show that several solvation shells around the weak acid molecule need to be included in the EVB model to correctly describe the stabilization of the solvated species. Structurally, one water molecule is coordinated to the proton donor in the protonated acid case, while two water molecules coordination is likely when the acid is deprotonated.

The dimer and trimer of BC2N are studied at the HF and MPn levels using 3-21G, 6-31G∗ and 6-31+G∗ basis sets. It is found that stability is enhanced by maintaining –BN– units together in the cyclic form. Both molecules have cyclic ground state structures with cumulenic-type equal bond lengths in the –BNBN– and polyacetylenic –CCCC– fragments. The linear–cyclic energy difference increases with the size of the cluster. Electron correlation substantially favors the cyclic structures over the linear forms. No general trend of atomic combination has been found in the linear isomers. Diffuse functions have negligible effects on the relative energies and geometries of the dimers.

A series of molecules related to malonaldehyde, containing an intramolecular H-bond, are used as the testbed for a variety of levels of ab initio calculation. Of particular interest are the excitation energies of the first set of valence excited states, nπ* and ππ*, both singlet and triplet, as well as the energetics of proton transfer in each state. Taking coupled cluster results as a point of reference, configuration interaction-singles–second-order Møller–Plesset (CIS–MP2) excitation energies are too large, as are CIS to a lesser extent, although these approaches successfully reproduce the order of the various states. The same may be said of complete active space self-consistent-field (CASSCF), which is surprisingly sensitive to the particular choice of orbitals included in the active space. Complete active space–second-order perturbation theory (CASPT2) excitation energies are rather close to coupled cluster singles and doubles (CCSD), as are density functional theory (DFT) values. CASSCF proton transfer barriers are large overestimates; the same is true of CIS to a lesser extent. MP2, CASPT2, and DFT barriers are closer to coupled cluster results, although yielding slight underestimates.

The ground and first few excited states of o-hydroxybenzaldehyde (oHBA) are computed at the CIS and MP2/CIS levels with a 6-31+G** basis set, with emphasis on its intramolecular H-bond. These results are compared with those for malonaldehyde, which differs from oHBA in that it lacks an adjoining benzene ring. In most respects, the addition of the latter aromatic system exerts surprisingly little influence upon the properties of malonaldehyde. With the exception of the 1ππ* state, electronic excitation weakens the H-bond and simultaneously raises the barrier to proton transfer in either system. Unlike the symmetric transfer potential in malonaldehyde, the enol and keto tautomers of oHBA are chemically distinct. π→π* excitation reverses the preference for the enol tautomer in the ground state. This reversal is connected with the changing degree of aromaticity in the benzene ring of oHBA. The asymmetric transfer potential in oHBA leads to forward and reverse barriers of different magnitude. When this factor is accounted for by an averaging procedure, the transfer barriers in oHBA are remarkably similar to those of the corresponding states of malonaldehyde.

The proton transfer from one oxygen atom to the other within the intramolecular H-bond in a molecule like o-hydroxybenzaldehyde (oHBA) would be precluded by a prior rotational isomerism that breaks this H-bond. The likelihood of such rotamerization in the ground and several excited electronic states is investigated by ab initio calculations at the CIS and MP2 levels with a 6-31+G** basis set. In the ground state, the energetics of proton transfer and rotamerization are competitive with one another; both processes are endothermic and must surmount an energy barrier. Excitation to the singlet or triplet pi pi* states presents a situation where tautomerization to the keto is exothermic, with a small barrier. In contrast. rotamerization is endothermic with high intervening barriers, so excited-state proton transfer is favored. The opposite situation is encountered in the n pi* states, where rotations of the hydroxyl and carbonyl groups are facile and lead energetically downhill, in contrast to the high barriers opposing endothermic tautomerization. The computations provide insights into the fundamental causes for the discrepancies between the behaviors of the pi pi* and n pi* states.

The central C atom of the OCCCO skeleton of the malonaldehyde molecule is replaced by N, and the effects upon the intramolecular H-bond and the proton transfer are monitored by ab initio calculations in the ground and excited electronic states. The H-bond is weakened in the singlet and triplet states arising from n --> pi(*) excitation in both molecules, which is accompanied by a heightened barrier to proton transfer.(3) pi pi(*) behaves in the same manner, but the singlet pi pi(*) state has a stronger H-bond and lower barrier. Replacement of the central C atom by N strengthens the intramolecular H-bond. Although the proton transfer barrier in the ground state of formimidol is lower than in malonaldehyde, the barriers in all four excited states are higher in the N-analog. The latter substitution also dampens the effect of the n --> pi(*) excitation upon the H-bond and increases the excitation energies of the various states, particularly pi pi(*). (C) 1998 John Wiley & Sons, Inc.

Tetra-atomic BC2N, the building block of the mixed carbon and BN clusters, has been studied at the HF, MP2, and CCSD(T) levels using both double- and triple-zeta basis sets with polarization and diffuse functions. In contrast to the parents C-4 and (BN)(2), the linear triplet ((3)Pi) BCCN is found to be the most stable and the linear-cyclic energy difference is about 28 kcal/mol. In the cyclic structure, isomers with adjacent B and N atoms are more stable, whereas no general trend of atomic combination has been found in the linear isomers. The preferred sequence of atoms in the linear form depends on the cluster size. The nature of bonding and atomization energies of the parent and hybrid molecules are compared and discussed.

The transfer of a proton in malonaldehyde takes place within an intramolecular II-bond involving a five-membered ring. This process is compared via ab initio methods with the transfer in analogous systems in which the size of the ring is altered to foul and to six and in which the system bears an overall negative charge. In addition to the ground state, calculations are applied to the singlet and triplet pi pi* states, as well as to (1)n pi* and (3)n pi*. The barriers to proton transfer are found to correlate strongly with various geometric and energetic markers of the strength of the II-bond. The II-bond is weakened by n --> pi* excitation, particularly for the neutral molecule, resulting in a higher transfer barrier. In the case of the two anions, excitation to (3) pi pi* strengthens the II-bond, while the result is more ambiguous for the (1) pi pi* state. This trend is reversed in malonaldehyde where the singlet is strengthened by the excitation and the triplet weakened. Some of these patterns are traced directly to the nature of the pertinent orbitals and the density shifts arising from the excitation.

Although the electronic contribution to the strength of a H-bond Is unaffected by isotopic substitution, the heavier mass of deuterium compared with protium lowers some of the vibrational frequencies in the complex. The binding energy of the complex, which includes zero-point and thermal vibrational energies, can thus be altered by several tenths of a kcal mol(-1) by H/D substitution. Ab initio calculations are used to analyze this phenomenon in a number of common organic functional groups that are prone to form H-bonds: hydroxyl, carbonyl, carboxyl and amide, both self complexing as homodimers and with water molecules as partners, It is found that any site of D-substitution increases the complexation energy; however, the bridging sites show a stronger preference for D over H than do the non-bridging, or terminal, sites, Hence D-bonding can be considered to be stronger than H bonding in these functional groups. Of the groups considered, the energetic preference for D over H Is greatest in the hydroxyl group, so deuterium would be expected to gravitate toward solvent water molecules in isotopic scrambling experiments. The increments in H-bonding energy resulting from each site of substitution are additive in cases of multiple substitution. (C) 1997 by John Wiley & Sons, Ltd.

The relative energies of H and D bonds are due to differences in zero-point vibrational energy (ZPVE). Ab initio calculations are used to assess the changes in this quantity that accompany all possible substitutions of protium by deuterium in a number of complexes. The ZPVE of the D bond is lower than that of the I-I bond in the neutral dimer and trimer of water. This difference can be traced to one particular vibrational mode, the one which displaces the bridging atom away from the O...O axis. The heavier mass of D lowers the frequency, and hence the ZPVE associated with it. The situation reverses itself in ionic H bonds. The total ZPVE of the (H2O..H..OH2)(+) complex is higher when a D occupies the bridging position, as compared to a terminal site. This difference is attributed to the intramolecular modes. Although replacement of the central H by D reduces the intermolecular ZPVE, the reduction of the intramolecular ZPVE is even larger when the substitution is made at a peripheral atom, so a D would tend to migrate away from a bridging location. This effect is noted also in the larger complex in which two methanol molecules are bound by a proton. The lower energy of a H bond as compared to a D bond is observed as well in the anionic (HOH..OH)(-) system, although the magnitude of the preference is smaller here. In all cases, raising the temperature, and thus invoking thermal vibrational and entropic effects, tends to preferentially stabilize H over D bonds.