We present an implementation of the time-dependent configuration-interaction singles (TDCIS) method for treating atomic strong-field processes. In order to absorb the photoelectron wave packet when it reaches the end of the spatial grid, we add to the exact nonrelativistic many-electron Hamiltonian a radial complex absorbing potential (CAP). We determine the orbitals for the TDCIS calculation by diagonalizing the sum of the Fock operator and the CAP using a flexible pseudospectral grid for the radial degree of freedom and spherical harmonics for the angular degrees of freedom. The CAP is chosen such that the occupied orbitals in the Hartree-Fock ground state remain unaffected. Within TDCIS, the many-electron wave packet is expanded in terms of the Hartree-Fock ground state and its single excitations. The virtual orbitals satisfy nonstandard orthogonality relations, which must be taken into consideration in the calculation of the dipole and Coulomb matrix elements required for the TDCIS equations of motion. We employ a stable propagation scheme derived by second-order finite differencing of the TDCIS equations of motion in the interaction picture and subsequent transformation to the Schrödinger picture. Using the TDCIS wave packet, we calculate the expectation value of the dipole acceleration and the reduced density matrix of the residual ion. The technique implemented will allow one to study electronic channel-coupling effects in strong-field processes.

The creation of superpositions of hole states via single-photon ionization using attosecond extremeultraviolet pulses is studied with the time-dependent configuration interaction singles (TDCIS) method. Specifically, the degree of coherence between hole states in atomic xenon is investigated. We find that interchannel coupling not only affects the hole populations, it also enhances the entanglement between the photoelectron and the remaining ion, thereby reducing the coherence within the ion. As a consequence, even if the spectral bandwidth of the ionizing pulse exceeds the energy splittings among the hole states involved, perfectly coherent hole wave packets cannot be formed. For sufficiently large spectral bandwidth, the coherence can only be increased by increasing the mean photon energy. The typical time scale of electronic motion in atoms, molecules, and condensed matter systems ranges from a few attoseconds (1 as = 10 −18 s) to tens of femtoseconds (1 fs = 10 −15 s) [1][2][3]. In the last decade the remarkable progress in high harmonic generation [4][5][6][7][8] made it possible to generate attosecond pulses as short as 80 as [9]. Attosecond pulses have opened the door to real-time observations of the most fundamental processes on the atomic scale [1,10]. For instance, the generation of attosecond pulses was utilized to determine spatial structures of molecular orbitals [11]; an interferometric technique using attosecond pulses was used to characterize attosecond electron wave packets [12]; and attosecond pulse trains [13] and isolated attosecond pulses [14], in combination with an intense few-cycle infrared pulse, enabled the control of electron localization in molecules. Attosecond technology demonstrated the ability to follow, on a subfemtosecond time scale, processes such as photoionization [15], Auger decay [16], and valence electron motion driven by relativistic spin-orbit coupling [17]. Furthermore, the availability of attosecond pulses fuelled a broad interest in exploring charge transfer dynamics following photoexcitation or photoionization [14].In this Letter, we analyze the creation of hole states via single-photon ionization using a single extremeultraviolet attosecond pulse. We investigate the impact of the freed photoelectron on the remaining ion and demonstrate that the interaction between the photoelectron and the ion cannot be neglected for currently available state-of-the-art attosecond pulses. In particular, the interchannel coupling of the initially coherently excited hole states greatly enhances the entanglement between the photoelectron and the ionic states. Interchannel coupling is mediated by the photoelectron and mixes different ionization channels, i.e., hole configurations, with each other. Consequently, the degree of coherence among the ionic states is strongly reduced, making it impossible to describe the subsequent charge transfer in the ion with a pure quantum mechanical state. Experiments on photosynthetic systems [18][19][20][21] have revealed a correlation between highly e...

Polyaromatic hydrocarbons (PAHs) are a class of organic molecules with importance in several branches of science, including medicine, combustion chemistry, and materials science. The delocalized π-orbital systems in PAHs require highly accurate electronic structure methods to capture strong electron correlation. Treating correlation in PAHs has been challenging because (i) traditional wave function methods for strong correlation have not been applicable since they scale exponentially in the number of strongly correlated orbitals, and (ii) alternative methods such as the density-matrix renormalization group and variational two-electron reduced density matrix (2-RDM) methods have not been applied beyond linear acene chains. In this paper we extend the earlier results from active-space variational 2-RDM theory [Gidofalvi, G.; Mazziotti, D. A. J. Chem. Phys. 2008, 129, 134108] to the more general two-dimensional arrangement of rings--acene sheets--to study the relationship between geometry and electron correlation in PAHs. The acene-sheet calculations, if performed with conventional wave function methods, would require wave function expansions with as many as 1.5 × 10(17) configuration state functions. To measure electron correlation, we employ several RDM-based metrics: (i) natural-orbital occupation numbers, (ii) the 1-RDM von Neumann entropy, (iii) the correlation energy per carbon atom, and (iv) the squared Frobenius norm of the cumulant 2-RDM. The results confirm a trend of increasing polyradical character with increasing molecular size previously observed in linear PAHs and reveal a corresponding trend in two-dimensional (arch-shaped) PAHs. Furthermore, in PAHs of similar size they show significant variations in correlation with geometry. PAHs with the strictly linear geometry (chains) exhibit more electron correlation than PAHs with nonlinear geometries (sheets).

The variational two-electron reduced-density-matrix (2-RDM) method, scaling polynomially with the size of the system, was applied to linear chains and three-dimensional clusters of atomic hydrogen as large as H(64). In the case of the 4x4x4 hydrogen lattice of 64 hydrogen atoms, a correct description of the dissociation requires about 10(18) equally weighted determinants in the wave function, which is too large for traditional multireference methods. The correct energy in the dissociation limit was obtained from the variational 2-RDM method in contrast to Hartree-Fock and single-reference methods. Analysis of the occupation numbers demonstrates that even for 1.0 A bond distances the presence of strong electron correlation requires a multireference method. Three-dimensional systems exhibit a marked increase in electron correlation from one-dimensional systems regardless of size. The metal-to-insulator transition upon expansion of the clusters was studied using the decay of the 1-RDM off-diagonal elements. The variational 2-RDM method was shown to capture the metal-to-insulator transition and dissociation behavior accurately for all systems.