A discussion of many of the recently implemented features of GAMESS (General Atomic and Molecular Electronic Structure System) and LibCChem (the C++ CPU/GPU library associated with GAMESS) is presented. These features include fragmentation methods such as the fragment molecular orbital, effective fragment potential and effective fragment molecular orbital methods, hybrid MPI/OpenMP approaches to Hartree–Fock, and resolution of the identity second order perturbation theory. Many new coupled cluster theory methods have been implemented in GAMESS, as have multiple levels of density functional/tight binding theory. The role of accelerators, especially graphical processing units, is discussed in the context of the new features of LibCChem, as it is the associated problem of power consumption as the power of computers increases dramatically. The process by which a complex program suite such as GAMESS is maintained and developed is considered. Future developments are briefly summarized.

The excitation spectra of linear atomic chains of silver and gold with various sizes have been calculated using time-dependent density functional theory. Silver chains show longitudinal and transverse peaks as well as a low-intensity d-band. The longitudinal peak, corresponding to the HOMO-LUMO transition (along the main axis of the chain), shifts linearly to the red as the length of the system increases, consistent with the particle-in-a-box model. The transverse peak remains at approximately constant energy for all systems studied and corresponds to ∑(m)→Π(m) transitions in the xy plane perpendicular to the chain. As the chain grows, transitions arising from d orbitals contribute to the transverse peak, which affects its oscillator strength. Contrary to silver, gold chains display a strong d-band that converges to a distinct pattern at a chain length of about twelve atoms. The transitions involved in the d-band originate from localized d-orbitals with a d(z(2)) character since they have the right symmetry to give transitions into the LUMO, LUMO + 1, …, which have ∑ symmetry. Transitions arising from these localized d-orbitals also affect the position of the longitudinal peak and generate a wide transverse band. Although the majority of the transitions involved in the transverse band have a d∑→Π or dΠ→∑ character, they are hidden by much stronger excitations of dΠ→Π character in gold nanowires.

The surface plasmon resonance (SPR) of noble metal nanoparticles is reviewed in terms of both classical and quantum mechanical approaches. The collective oscillation of the free electrons responsible for the plasmon is well described using classical electromagnetic theory for large systems (from about 10 to 100 nm). In cases where quantum effects are important, this theory fails and first principle approaches like time-dependent density functional theory (TDDFT) must be used. In this paper, we give an account of the current understanding of the quantum mechanical origin of plasmon resonances. We provide some insight into how the discrete absorption spectrum of small noble metal clusters evolves into a strong plasmon peak with increasing particle size. The collective character of the plasmon is described in terms of the constructive addition of single-particle excitations. As the system size increases, the number of single-particle excitations increases as well. A configuration interaction (CI) approach can be applied to describe the optical properties of particles of all shapes and sizes, providing a consistent definition of plasmon resonances. Finally, we expand our analysis to thiolate-protected nanoparticles and analyze the effects of ligands on the plasmon.

The origin of plasmon resonance in acenes is described by analyzing the excitation spectrum of naphthalene in terms of configuration interaction. The strong longitudinal β-peak in the UV region of the spectrum results from a constructive interaction of the transition dipole moments of two degenerate configurations V 1 and V 2. V 1 corresponds to the excitation of an electron from the HOMO to the LUMO+1. V 2 corresponds to the excitation of an electron from the HOMO-1 to the LUMO. The weak longitudinal α-peak in the visible results from a destructive interaction of the dipole moments of the same two configurations. Previous time-dependent density functional theory (TDDFT) calculations showed a similar behavior for silver and gold nanoparticles but often with more than two interacting configurations. The plasmon occurs at the frequency where all configurations interact constructively. The β-peak of acenes can therefore be identified as the plasmon peak. The natural transition orbitals involved in the α- and β- peaks of naphthalene have identical shapes, which reflects the fact that the transitions involved in these two peaks are similar, but they may have opposite phases. An analysis of the transition density of the β-peak of naphthalene reveals that the electron density moves from one side of the molecule to the other upon excitation, as expected for a dipolar plasmon. The plasmonic character of the β-peak is compared to the single-particle transition character of the transverse p-band. Several exchange-correlation functionals have been benchmarked. Hybrid functionals give the best description of the β-peak and the α-peak. The couplings between the two interacting configurations at all levels of theory are similar to experimental values. On the other hand, long-range corrected functionals give the most accurate energies for the transverse p-band.

The effect of silver doping of the Au 25 (SH) 18 − nanoparticle is studied by investigating Au 25−n Ag n (SH) 18 − (n = 1, 2, 4, 6, 8, 10, 12) systems using DFT. For n = 1, doping of the icosahedral shell of the metal core is energetically more favorable than doping of the metal−thiolate units or the center of the core. For n ≥ 2, only doping of the core surface is considered, and arrangements where the silver dopants are in close proximity tend to be slightly less favorable. However, energy differences are small, and all conformations are accessible under experimental conditions. Boltzmann-averaged excitation spectra for these systems show similar features to the undoped Au 25 (SH) 18 −. The main differences include a blue shift of the low-energy HOMO−LUMO (1P→1D) peak and an increased intensity of the peak at 2.5 eV as the number of doping silver atoms increases. Silver doping lowers the energy of ligand-based orbitals and facilitates the transitions between the superatom orbitals. Silver-doped systems show broader excitation spectra due to a breaking of the symmetry of the superatom orbitals.

A plasmon-like phenomenon, arising from coinciding resonant excitations of different electronic characteristics in 1D silver nanowires, has been proposed based on theoretical linear absorption spectra. Such a molecular plasmon holds the potential for anisotropic nanoplasmonic applications. However, its dynamical nature remains unexplored. In this work, quantum dynamics of longitudinal and transverse excitations in 1D silver nanowires are carried out within the real-time time-dependent density functional theory framework. The anisotropic electron dynamics confirm that the transverse transitions of different electronic characteristics are collective in nature and oscillate in-phase with respect to each other. Analysis of the time evolutions of participating one-electron wave functions suggests that the transverse transitions form a coherent wave packet that gives rise to a strong plasmon resonance at the molecular level. © 2014 AIP Publishing LLC. [http://dx

An analysis of the excitation spectra of silver and gold nanorods with different cross sections, lengths, and diameters was performed using time-dependent density functional theory at the LB94/ DZ level. Silver nanorods show a strong longitudinal peak, corresponding to excitations along the main axis (z axis) of the nanorods, and a smaller transverse peak, corresponding to excitations in the xy plane of the nanorods. For systems with a large cross-section (star-shaped and large pentagonal nanorods), the single transverse peak is split into a wide band. The orbitals involved in these transitions are delocalized cylindrical orbitals. Constructive addition of the dipole moments of these transitions is observed for the strong longitudinal and transverse peaks, which is likely at the origin of the surface plasmon resonance phenomenon. The wavelength of the longitudinal peak increases linearly with increasing length, crossing over the transverse peak or transverse band, which remains at nearly constant energy and intensity. The intensity of the longitudinal peak increases with increasing system length due to the increasing number of electrons being collectively excited. The energy of the longitudinal peak for systems of identical length also tends to increase as the diameter of the system increases, which can be correlated to a decreasing aspect ratio. Gold nanorods display more complex excitation spectra due to the presence of transitions originating from the d-band. Such transitions may also mix with cylindrical orbital-based transitions, especially for systems with low aspect ratios, splitting the longitudinal peak into several peaks of lower intensity. As the aspect ratio increases, the energy of the longitudinal peak decreases, and its intensity increases. It then becomes separated from the dband transitions which remain approximately constant in intensity and energy. Consequently, the amount of d-band coupling to the main cylindrical orbital-based excitations decreases, which leads to a strong isolated longitudinal peak similar to the silver case. No strong transverse peak is observed for gold nanorods at this level of theory. Instead, the transverse excitations are hidden by the d-band transitions.

It is often desirable to accurately and efficiently model the behavior of large molecular systems in the condensed phase (thousands to tens of thousands of atoms) over long time scales (from nanoseconds to milliseconds). In these cases, ab initio methods are difficult due to the increasing computational cost with the number of electrons. A more computationally attractive alternative is to perform the simulations at the atomic level using a parameterized function to model the electronic energy. Many empirical force fields have been developed for this purpose. However, the functions that are used to model interatomic and intermolecular interactions contain many fitted parameters obtained from selected model systems, and such classical force fields cannot properly simulate important electronic effects. Furthermore, while such force fields are computationally affordable, they are not reliable when applied to systems that differ significantly from those used in their parameterization. They also cannot provide the information necessary to analyze the interactions that occur in the system, making the systematic improvement of the functional forms that are used difficult. Ab initio force field methods aim to combine the merits of both types of methods. The ideal ab initio force fields are built on first principles and require no fitted parameters. Ab initio force field methods surveyed in this perspective are based on fragmentation approaches and intermolecular perturbation theory. This perspective summarizes their theoretical foundation, key components in their formulation, and discusses key aspects of these methods such as accuracy and formal computational cost. The ab initio force fields considered here were developed for different targets, and this perspective also aims to provide a balanced presentation of their strengths and shortcomings. Finally, this perspective suggests some future directions for this actively developing area.