Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H10: Focus Session: Frontiers in Computational Chemical Physics I |
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Sponsoring Units: DCP Chair: Eugene Mele, University of Pennsylvania Room: Baltimore Convention Center 302 |
Tuesday, March 14, 2006 11:15AM - 11:51AM |
H10.00001: A Revisit to Soliton Theory Invited Speaker: Our recent studies on the soliton theory of conjugated polymers, via Hubbard model and first-principles computations, reveal two subtle points concerning electron-electron correlation and electron-phonon coupling. Significant three-dimensional conformational changes and a sequence of self-localized electronic states are identified and attributed to the presence of soliton. The soliton-induced conformational deformations provide an intrinsic high strain-rate actuation mechanism in optical excitation processes, while the sensitivity of new self-localized states to the presence of local fields provides a high-resolution sensing mechanism for ions and radicals. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H10.00002: Auxiliary field quantum Monte Carlo with a localized basis--applications to atoms and molecules Wissam A. Al-Saidi, Shiwei Zhang, Henry Krakauer We extended the recently introduced phaseless auxiliary field quantum Monte Carlo approach [1] to any single-particle basis, and applied it to study atoms and molecules using localized Gaussian basis. This method maps the interacting many-body problem into a linear combination of non-interacting problems using a complex Hubbard-Stratonovich transformation, and controls the phase/sign problem using a trial wave function. It employs a random walk approach in Slater determinant space to project the many-body ground state of the system. The computational cost scales as a low power of system size. In all of the presented results the trial wave function was from a Hartree-Fock calculation. The obtained total energies of the atoms and molecules agree to within a few milli Hartrees with the exact value from full configuration interaction or density matrix renormalization group. The results are comparable in accuracy to those of CCSD(T) for equilibrium geometries but are superior for bond breaking. [1] S. Zhang and H. Krakauer, Phys. Rev. Lett. {\bf 90}, 136401 (2003). [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H10.00003: Monte Carlo Studies of Soft Pentagonal Rotors and Polymer Self-Assembly Michael Winokur, Marcus Mueller Side chain substitution is widely used in $\pi$-conjugated polymers to promote processibility and reduce interchain aggregation. However this functionalization also creates diverse materials that self-organize at molecular length scales. Small modifications in the chemical architecture often produce large variations in the observed structures and phase behavior. An example of this is poly(9,9-bis(2-ethylhexyl)fluorene) (PF2/6) in which single chains form helices that subsequently undergo a supramolecular self-assembly to give a three chain unit cell or, equivalently, a $\sqrt{3}\times\sqrt{3}$ superlattice. Direct structure studies indicate that PF2/6 incorporates a heterogeneous sequence of conformational isomers approximating a 5-fold helix and this construction defines the overall shape of the chain. In terms of more conventional polymers we consider PF2/6 to be representative of a sparse brush with, on average, a columnar type structure. However PF2/6 structure studies already indicate that the potential is not uniformly cylindrical because of the 5-fold helix and the spontaneous formation of a 3 chain unit cell. Through construction of a representative ansatz Hamilton we have mimicked the interaction between soft pentagonal shaped rotors. Monte Carlo calculations employing this Hamiltonian identify a range of parameters that reproduce the experimentally observed $\sqrt{3}\times\sqrt{3}$ superlattice. Moreover we find that small changes in this orientational pair potential strongly alter the phase behavior. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H10.00004: Particle-Hole Corrections for Accurate DFT-based Charge Transfer Energetics Tunna Baruah, M. Pederson Density functional theory (DFT) is primarily a ground state theory because it lacks a rigorous prescription for the calculation of excited states. However, a formalism due to Theophilou showed that a Hohenberg-Kohn like theorem is extendable to an ensemble of ground and excited states provided the states are mutually orthogonality. This condition is also a necessary for a variational approach to individual excited states as shown by Levy. We present a way for obtaining accurate single-electron excitation energies from the ground-state Hamiltonian and orbitals. Our approach explicitly guarantees mutual orthogonality and removes spurious interactions between the electron and hole states. The method yields excellent results for inert gas atoms, closed-shell atoms and molecules, and to charge transfer systems such as the dissociated Na-Cl molecule and a 50 nm long organic molecule containing 200 atoms. Initial applications rely entirely upon standard GGA functionals but the approach is equally amenable to any mean-field method such as Hartree-Fock or DFT with SIC. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H10.00005: Low Energy Inelastic Helium Atom Scattering from Monolayers L.W. Bruch, F.Y. Hansen A time-dependent wave packet calculation for inelastic low energy helium atom scattering (HAS) by a physisorbed monolayer\footnote{L. W. Bruch and F. Y. Hansen, J. Chem. Phys. {\bf 122}, 114714 (2005)} is extended to much longer propagation times by adding an absorbing potential at large distances. This enables a treatment of transiently trapped helium atoms for He/Xe/Pt(111) and shows that the lifetimes are in the range 10-30 ps. The scattering of the wave packet is essentially complete when the propagation is terminated. Systematic trends for an experiment on a Xe/Pt(111) monolayer\footnote{L. W. Bruch, A. P. Graham, and J. P. Toennies, J. Chem. Phys. {\bf 112},314 (2000)} are discussed. The remarkable inelastic intensity for the shear horizontal (SH) monolayer branch is predicted to be strongly enhanced for incident energies in the range 4 to 6 meV, somewhat below the 8.2 meV energy used in most of that experiment. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 1:15PM |
H10.00006: A time-dependent density functional theory approach for the excited state dynamics of nanostructures and biomolecules Invited Speaker: We will review the recent implementations of TDDFT to study the optical absoprtion of biological chromophores, one-dimensional polymers and layered materials. In particular we will show the effect of electron-hole attraction in those systems. We will folow two routes: one based on solving the Bethe-Salpeter equation and the other on an orbital-dependent OEP method on top of the GW approximation for the self-energy. Virtues and deficiencies of both methods will be illustrated. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H10.00007: Energetic, structural and vibrational properties of carbon monoxide on transition-metal surfaces: a linear-response approach to the energy puzzle Ismaila Dabo, Andrzej Wieckowsi, Nicola Marzari We have studied the energetic, structural and vibrational properties of carbon monoxide adsorbed on platinum and platinum-ruthenium surfaces using density-functional theory within the generalized-gradient approximation (GGA) and its molecular U variant (GGA + molecular U) introduced by Kresse {\it et al. [Phys. Rev. B 68, 73401 (2003)]}. Our study puts in evidence that the GGA frequency predictions for the C-O stretching mode are in excellent agreement with spectroscopic measurements. This agreement is made more remarkable by the generally-poor performance of standard exchange-correlation functionals which fail even in predicting the most stable adsorption site for CO on platinum. It is also shown that the molecular U correction to the electronic hybridizations and adsorption energies does not affect the excellent accuracy of the GGA vibrational-property predictions. Based on the linear-response approach proposed by Cococcioni and de Gironcoli { \it [Phys. Rev. B 71, 35105 (2005)]}, we provide a theoretical justification for this observation. The accurary of density-functional theory in predicting accurate vibrational spectra while paradoxically failing in determining correct adsorption energies is also discussed. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H10.00008: y-induced chaotic Whispering Gallery modes Araceli Gongora-Trevino, Stuart Schaffner, Jorge Jose A free particle inside a circular billiard bouncing elastically from the boundary produces classically bounded caustic orbits and Whispering Gallery (WG) modes in the quantum case. If a sector is taken out from the circle, forming a Pacman-like billiard, the classical caustic orbits remain. However, in the quantum case the WG modes have fractional quantum angular momentum described by Bessel functions of fractional order. In this work we have considered the case when the straight walls of the Pacman are not fixed in time but oscillate periodically or non-periodically. The structure of the WG modes changes; there are cases where the WG mode intensity increases significantly in certain spatial locations when compared with the situation in the Pacman stationary case. This intensity increase is analyzed in detail, and it may be implemented and tested in the type of micro-cavity used in quantum dot lasers. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H10.00009: A generalization of the charge equilibration method Razvan A. Nistor, Jeliazko G. Polihronov, Martin H. Muser, Nick J. Mosey Charge-equilibration (C-Eq) methods are an efficient means of calculating the effective partial charges of the atoms in molecules. The aim of obtaining the partial charges is to reconstruct the electrostatic field of a molecule, which then may be used as an accurate replacement to {\it ab initio} interatomic potentials. However, the inability of the C-Eq method to consistently predict the correct charges for a wide range of molecular geometries has limited its potential use in molecular simulations. A generalization to a split charge (split-Q) formalism, where the semi-empirical fit parameters are defined not in terms of atom-type, but in terms of bond-type, greatly improves the quality, and more importantly, the transferability of the fits. The flexible formalism of the split-Q approach allows for a number of generalizations over traditional C-Eq methods at the cost of additional fit parameters in the model. However, the split-Q approach can reproduce the charges on a variety of different molecules containing H, C, O, and Si, up to three times more accurately than previous methods when comparing to {\it ab initio} calculations. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H10.00010: Atomization energies using analytic density functional theory Rajendra Zope, Brett Dunlap Our recent formulation of analytic and variational Slater-Roothaan (SR) method uses Gaussian basis sets to express the molecular orbitals, electron density and the one-body effective potential of density functional theory. It allows arbitrary scaling of the exchange potential around each atom in heteroatomic systems. We examine the performance of the SR method for atomization energies using values of Slater’s scaling parameters determined from the Hartree-Fock and the exact total atomic energies. We also present another set of scaling parameters for atomization energies. The performance of the computationally efficient SR method for the atomization energies of a set of 148 molecules using this third set of parameters is comparable to that of the Perdew-Burke-Ernzerhof generalized gradient approximation. [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H10.00011: Comparison of two simple models for high frequency friction: Exponential vs. Gaussian wings Steven Adelman We describe new methods for ruling out unphysical forms for the high frequency friction $_{\omega \to \infty }^{\lim } \quad \beta (\omega )$ needed to compute vibrational energy relaxation times. These are based on the fluctuating force autocorrelation function$(faf)\mbox{ }C(t)=\langle \tilde {\Im }^2\rangle _0 ^{-1}\langle \tilde {\Im }(t)\tilde {\Im }\rangle _0 ,$which is proportional to the Fourier transform of $\beta (\omega )$. Here we compare two model faf's C$_{se}$(t) = sech (t/$\tau )$ and C$_{ga}$(t) = exp$\left[ {-\frac{1}{2}\left( {\frac{t}{\tau }} \right)^2} \right]\mbox{ }$. These give respective high frequency frictions which have incompatible exponential and Gaussian forms. We apply our procedures to eliminate C$_{se}$(t). We do this by showing from $\beta _{se}(\omega ) \quad \equiv $ $\frac{\langle \Im ^2\rangle _0 }{k_B T}\int_0^\infty {\cos \omega t} $C$_{se}$(t)dt that $_{\omega \to \infty }^{\lim } \beta _{se}(\omega )$ derives from the long time ``tail'' of C$_{se}$(t). We then note that C$_{se}$(t) is built only from short time quantities, rendering the form of this ``tail'' artifactual. Thus the exponential form of $_{\omega \to \infty }^{\lim } \beta (\omega )$, is also artifactual. [Preview Abstract] |
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