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Yousung Jung

Publications and source records attributed to Yousung Jung.

6 recordsLinked to original sources
PubMed2005

Auxiliary basis expansions for large-scale electronic structure calculations.

One way to reduce the computational cost of electronic structure calculations is to use auxiliary basis expansions to approximate four-center integrals in terms of two- and three-center integrals, usually by using the variationally optimum Coulomb metric to determine the expansion coefficients. However, the long-range decay behavior of the auxiliary basis expansion coefficients has not been characterized. We find that this decay can be surprisingly slow. Numerical experiments on linear alkanes and a toy model both show that the decay can be as slow as 1/r in the distance between the auxiliary function and the fitted charge distribution. The Coulomb metric fitting equations also involve divergent matrix elements for extended systems treated with periodic boundary conditions. An attenuated Coulomb metric that is short-range can eliminate these oddities without substantially degrading calculated relative energies. The sparsity of the fit coefficients is assessed on simple hydrocarbon molecules and shows quite early onset of linear growth in the number of significant coefficients with system size using the attenuated Coulomb metric. Hence it is possible to design linear scaling auxiliary basis methods without additional approximations to treat large systems.

Alkanes↗
PubMed2005

Cycloaddition of benzene on Si(100) and its surface conversions.

A comprehensive ab initio study of the adsorption of benzene on the silicon(100) surface is presented. Five potential candidates ([2+2] adduct, [4+2] adduct, two tetra-sigma-bonded structures, and one radical-like structure) for the reaction product are examined to determine the lowest energy adsorption configuration. A [4+2] butterfly structure is determined to be the global minimum (-29.0 kcal/mol), although one of the two tetra-sigma-bonded structures (-26.7 kcal/mol) is similar in energy to it. Multireference perturbation theory suggests that the [4+2] addition mechanism of benzene on Si(100) is very similar to the usual Diels-Alder reaction (i.e., small or zero activation barrier), even though benzene adsorption entails the loss of benzene aromaticity during the reaction. On the other hand, the [2+2] cycloaddition mechanism is shown to require a relatively high activation barrier (17.8 kcal/mol), in which the initial step is to form a (relatively strongly bound) van der Waals complex (-8.9 kcal/mol). However, the net activation barrier relative to reactants is only 8.9 kcal/mol. Careful examination of the interconversion reactions among the reaction products indicates that the two tetra-sigma-bonded structures (that are energetically comparable to the [4+2] product) can be derived from the [2+2] adduct with activation barriers of 15.5 and 21.4 kcal/mol. However, unlike the previous theoretical predictions, it is found that the conversion of the [4+2] product to the tetra-sigma-bonded structures entails huge barriers (>37.0 kcal/mol) and is unlikely to occur. This suggests that the [4+2] product is not only thermodynamically the most stable configuration (lowest energy product) but also kinetically very stable (large barriers with respect to the isomerization to other products).

Journal Article↗
PubMed2004

Scaled opposite-spin second order Møller-Plesset correlation energy: an economical electronic structure method.

A simplified approach to treating the electron correlation energy is suggested in which only the alpha-beta component of the second order Møller-Plesset energy is evaluated, and then scaled by an empirical factor which is suggested to be 1.3. This scaled opposite-spin second order energy (SOS-MP2), where MP2 is Møller-Plesset theory, yields results for relative energies and derivative properties that are statistically improved over the conventional MP2 method. Furthermore, the SOS-MP2 energy can be evaluated without the fifth order computational steps associated with MP2 theory, even without exploiting any spatial locality. A fourth order algorithm is given for evaluating the opposite spin MP2 energy using auxiliary basis expansions, and a Laplace approach, and timing comparisons are given.

Algorithms↗
PubMed2004

Intermolecular pi-to-pi bonding between stacked aromatic dyads. Experimental and theoretical binding energies and near-IR optical transitions for phenalenyl radical/radical versus radical/cation dimerizations.

The high symmetry and stability of phenalenyl systems, both as the planar pi-radical (P*) and as the pi-cation (P+), are desirable characteristics of prototypical aromatic donor/acceptor pairs that encourage their use as (binary) models for the study of intermolecular interactions extant in stacked molecular arrays. Thus, quantitative ESR spectroscopy of the paramagnetic P* identifies its spontaneous self-association to the diamagnetic P2, previously characterized as the stacked pi-dimer by X-ray crystallography. Likewise, the rapid cross-association of P* with the closed-shell P+ leads to the stacked pi-dimer cation P2*+ with the "doubled" ESR spectrum diagnostic of complete (odd) electron delocalization. These pi-associations are confirmed by UV-vis studies that reveal diagnostic near-IR bands of both P2 and P2*+-strongly reminiscent of intermolecular charge-transfer absorptions in related aromatic (donor/acceptor) pi-associations. Ab initio molecular-orbital calculations for the pi-dimer P2 predict a binding energy of DeltaED = -11 kcal mol(-1), which is in accord with the experimental enthalpy change of DeltaHD = -9.5 kcal mol(-1) in dichloromethane solution. Most importantly, the calculations reproduce the intermonomer spacings and reveal the delicate interplay of attractive covalent and dispersion forces, balanced against the repulsions between filled orbitals. For comparison, the binding energy in the structurally related cationic pi-pimer P2*+ is calculated to be significantly larger with DeltaEP approximately -20 kcal mol(-1) (gas phase), owing to favorable electrostatic interactions not present in the neutral pi-dimer (which outweigh the partial loss of covalent interactions). As a result, our theoretical formulation can correctly account for the experimental enthalpy change in solution of DeltaHP = -6.5 kcal mol(-1) by the inclusion of differential ionic solvation in the formation of the pi-pimer.

Journal Article↗
PubMed2004

Aromaticity of four-membered-ring 6pi-electron systems: N2S2 and Li2C4H4.

N(2)S(2) is a four-membered-ring system with 6pi electrons. While earlier proposals considered N(2)S(2) to be aromatic, recent electronic structure calculations claimed that N(2)S(2) is a singlet diradical. Our careful reexamination does not support this assertion. N(2)S(2) is closed shell and aromatic since it satisfies all three generally accepted criteria for aromaticity: energetic (stability), structural (planarity with equal bond lengths), and magnetic (negative nucleus-independent chemical shift due to the pi electrons). These characteristics as well as the electronic structure of N(2)S(2) are compared with those for an isoelectronic pi system, Li(2)C(4)H(4), motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N(2)S(2) and Li(2)C(4)H(4) are both essentially 2pi-electron aromatic systems with a formal N-S (C-C) bond order of 1.25 even though they both have 6pi electrons. This is because four of the six pi electrons occupy the nonbonding pi HOMOs and only two electrons participate effectively in the aromatic stabilization. However, wave function analysis shows relatively large LUMO occupation numbers; this antibonding effect can be said to reduce the aromatic character by approximately 7% and 4% for N(2)S(2) and Li(2)C(4)H(4), respectively.

Journal Article↗