Multireference Single Double Configuration Interaction Research Articles

Graphene Quantum Dot with Divacancy and Topological Defects: A Novel Material for Promoting Prompt and Delayed Fluorescence of Tunable Wavelengths

This work demonstrates the unique approach of introducing divacancy imperfections in topological Stone-Wales type defected graphene quantum dots for harvesting both singlet and triplet excitons, essential for fabricating fluorescent organic light-emitting diodes. Here, we first reveal that structural relaxation of these systems establishes the high-spin triplet state as the stable ground state at room temperature, thereby significantly increasing their potential in designing spintronic devices. Our extensive electron-correlated computations then demonstrate that the energetic ordering of the singlet and triplet states in these relaxed structures can trigger both prompt and delayed fluorescence of different wavelengths through various decay channels. Particularly, the position of divacancy determines the tunability range of the emission wavelengths. In addition, our results obtained from both multi-reference singles-doubles configurationinteraction (MRSDCI) and first-principles time-dependent density functional theory (TDDFT) methodologies highlight that the synergetic effects of divacancy-position,structural relaxation and spin multiplicity critically govern the nature and magnitude of shift exhibited by the most intense peak of the absorption profile, crucial for designing optoelectronic devices.

Pariser–Parr–Pople Model Based Configuration-Interaction Study of Linear Optical Absorption in Lower-Symmetry Polycyclic Aromatic Hydrocarbon Molecules

The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with lower symmetry, namely, benzo[ghi]perylene (C22H12), benzo[a]coronene (C28H14), naphtho[2,3a]coronene (C32H16), anthra[2,3a]coronene (C36H18), and naphtho[8,1,2-abc]coronene (C30H14), were investigated. We performed electron-correlated calculations using screened and standard parameters in the π-electron Pariser–Parr–Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using multireference singles–doubles configuration-interaction (MRSDCI) methodology. The PPP model Hamiltonian includes long-range Coulomb interactions, which increase the accuracy of our calculations. The results of our calculations predict that, with the increasing sizes of the coronene derivatives, optical spectra are red-shifted, and the optical gaps decrease. In each spectrum, the first peak representing the optical gap is of moderate intensity, while the more intense peaks appear at higher energies. Our computed spectra are in good agreement with the available experimental data. For the purpose of comparison, we also performed first-principles time-dependent density-functional theory (TDDFT) calculations of the optical gaps of these molecules using Gaussian basis functions and found that they yielded values lower than our configuration-interaction (CI) results.

Systematic First-Principles Configuration-Interaction Calculations of Linear Optical Absorption Spectra in Silicon Hydrides: Si2H2n (n = 1-3).

We have performed first-principles electron-correlated calculations employing large basis sets to optimize the geometries and to compute linear optical absorption spectra of various low-lying conformers of silicon hydrides: Si2H2n, n = 1, 2, 3. The geometry optimization for various isomers was carried out at the coupled-cluster singles-doubles-perturbative-triples [CCSD(T)] level of theory, while their excited states and absorption spectra were computed using a large-scale multireference singles-doubles configuration-interaction approach, which includes electron-correlation effects at a sophisticated level. Our calculated spectra are the first ones for Si2H2 and Si2H4 conformers, while for Si2H6, we obtain excellent agreement with the experimental measurements, suggesting that our computational approach is reliable. Our calculated absorption spectra exhibit a strong structure-property relationship, suggesting the possibility of identifying various conformers based on their optical absorption fingerprints. Furthermore, we have also performed geometry optimization for the selected optically excited states, providing insights into their character.

Excited States and Optical Properties of Hydrogen-Passivated Rectangular Graphenes: A Computational Study

In this paper, we perform large-scale electron-correlated calculations of optoelectronic properties of rectangular graphene-like polycyclic aromatic hydrocarbon molecules. Theoretical methodology employed in this work is based upon Pariser-Parr-Pople (PPP) π-electron model Hamiltonian, which includes long-range electron-electron interactions. Electron-correlation effects were incorporated using multi-reference singles-doubles configurationinteraction (MRSDCI) method, and the ground and excited state wave functions thus obtained were employed to calculate the linear optical absorption spectra of these molecules, within the electric-dipole approximation. As far as the ground state wave functions of these molecules are concerned, we find that with the increasing size, they develop a strong diradical open-shell character. Our results on optical absorption spectra are in very good agreement with the available experimental results, outlining the importance of electron-correlation effects in accurate description of the excited states. In addition to the optical gap, spin gap of each molecule was also computed using the same methodology. Calculated spin gaps exhibit a decreasing trend with the increasing sizes of the molecules, suggesting that the infinite graphene has a vanishing spin gap.

First principles study of structural and optical properties of [formula omitted] isomers

In this work we undertake a comprehensive numerical study of the ground state structures and optical absorption spectra of isomers of B12 cluster. Geometry optimization was performed at the coupled-cluster-singles-doubles (CCSD) level of theory, employing cc-pVDZ extended basis sets. Once the geometry of a given isomer was optimized, its ground state energy was calculated more accurately at the coupled-cluster-singles-doubles along with perturbative treatment of triples (CCSD(T)) level of theory, employing larger cc-pVTZ basis sets. Thus, our computed values of binding energies of various isomers are expected to be quite accurate. Our geometry optimization reveals eleven distinct isomers, along with their point group, and electronic ground state symmetries. We also performed vibrational frequency analysis on the three lowest energy isomers, and found them to be stable. Therefore, we computed the linear optical absorption spectra of these isomers of B12, employing large-scale multi-reference singles-doubles configuration-interaction (MRSDCI) approach, and found a strong structure-property relationship. This implies that the spectral fingerprints of the geometries can be utilized for optical detection, and characterization, of various isomers of B12. We also explored the stability of the isomer with the structure of a perfect icosahedron, with Ih symmetry. In bulk boron icosahedron is the basic structural unit, but, our vibrational frequency analysis reveals that it is unstable in the isolated form. We speculate that this instability could be due to Jahn-Teller distortion because five-fold degenerate HOMO orbitals in Ih structure are unfilled.

First principles electron-correlated calculations of optical absorption in magnesium clusters

In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of various isomers of magnesium clusters Mg$_{n}$ (n=2--5), corresponding to valence transitions. Geometry optimization of several low-lying isomers of each cluster was carried out using coupled-cluster singles doubles (CCSD) approach, and these geometries were subsequently employed to perform ground and excited state calculations using either the full-CI (FCI) or the multi-reference singles-doubles configuration interaction (MRSDCI) approach, within the frozen-core approximation. Our calculated photoabsorption spectrum of magnesium dimer (Mg$_{2}$) isomer is in excellent agreement with the experiments both for peak positions, and intensities. Owing to the sufficiently inclusive electron-correlation effects, these results can serve as benchmarks against which future experiments, as well as calculations performed using other theoretical approaches, can be tested.

Large-scale first principles configuration interaction calculations of optical absorption in aluminum clusters.

We report the linear optical absorption spectra of aluminum clusters Aln (n = 2-5) involving valence transitions, computed using the large-scale all-electron configuration interaction (CI) methodology. Several low-lying isomers of each cluster were considered, and their geometries were optimized at the coupled-cluster singles-doubles (CCSD) level of theory. With these optimized ground-state geometries, excited states of different clusters were computed using the multi-reference singles-doubles configuration-interaction (MRSDCI) approach, which includes electron correlation effects at a sophisticated level. These CI wave functions were used to compute the transition dipole matrix elements connecting the ground and various excited states of different clusters, and thus their photoabsorption spectra. The convergence of our results with respect to the basis sets, and the size of the CI expansion, was carefully examined. Our results were found to be significantly different as compared to those obtained using time-dependent density functional theory (TDDFT) [Deshpande et al. Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 68, 035428]. When compared to the available experimental data for the isomers of Al2 and Al3, our results are in very good agreement as far as important peak positions are concerned. The contribution of configurations to many body wave functions of various excited states suggests that in most cases optical excitations involved are collective, and plasmonic in nature.

Pariser–Parr–Pople Model Based Investigation of Ground and Low-Lying Excited States of Long Acenes

Several years ago, Angliker et al. [ Chem. Phys. Lett. 1982 , 87 , 208 ] predicted nonacene to be the first linear acene with the triplet state 1(3)B2u as the ground state, instead of the singlet 1(1)Ag state. However, contrary to that prediction, in a recent experimental work, Tönshoff and Bettinger [ Angew. Chem. Int. Ed. 2010 , 49 , 4125 ] demonstrated that nonacene has a singlet ground state. Motivated by this experimental finding, we decided to perform a systematic theoretical investigation of the nature of the ground and the low-lying excited states of long acenes, with an emphasis on the singlet-triplet gap, starting from naphthalene, all the way up to decacene. The methodology adopted in our work is based upon the Pariser-Parr-Pople model (PPP) Hamiltonian, along with the large-scale multireference singles-doubles configuration interaction (MRSDCI) approach. Our results predict that even though the singlet-triplet gap decreases with the increasing conjugation length, nevertheless, it remains finite till decacene, thus providing no evidence of the predicted singlet-triplet crossover. We also analyze the nature of many-particle wave function of the correlated singlet ground state and find that the longer acenes exhibit a tendency toward an open-shell singlet ground state. Moreover, when we compare the experimental absorption spectra of octacene and nonacene with their calculated singlet and triplet absorption spectra, we observe excellent agreement for the singlet case. Hence, the optical absorption results also confirm the singlet nature of the ground state for longer acenes. Calculated triplet absorption spectra of acenes predict two well-separated intense long-axis polarized absorptions, against one such peak observed for the singlet case. This is an important prediction regarding the triplet optics of acenes, which can be tested in future experiments on oriented samples.

Correlated theory of linear optical absorption of octacene and nonacene

The technological importance of higher acenes has led to resurgence of interest in synthesizing higher acenes such as octacene, nonacene etc. Recently, Tönshoff and Bettinger [2010 Angew. Chem. Int. Ed. 49 4125] have synthesized octacene and nonacene. Motivated by their work, we have performed large-scale calculations of linear optical absorption of octacene and nonacene. Methodology adopted in our work is based upon Pariser-Parr-Pople model (PPP) Hamiltonian, along with large-scale multi-reference singles-doubles configuration interaction (MRSDCI) approach.

LARGE-SCALE FIRST PRINCIPLES CONFIGURATION INTERACTION CALCULATIONS OF OPTICAL ABSORPTION IN BORON CLUSTERS

We have performed systematic large-scale all-electron correlated calculations on boron clusters B n(n = 2 - 5), to study their linear optical absorption spectra. Several possible isomers of each cluster were considered, and their geometries were optimized at the coupled-cluster singles doubles (CCSD) level of theory. Using the optimized ground-state geometries, the excited states of different clusters were computed using the multi-reference singles-doubles configuration–interaction (MRSDCI) approach, which includes electron correlation effects at a sophisticated level. These CI wave functions were used to compute the transition dipole matrix elements connecting the ground and various excited states of different clusters, eventually leading to their linear absorption spectra. The convergence of our results with respect to the basis sets, and the size of the CI expansion were carefully examined. The contribution of configurations to many body wave-function of various excited states suggests that the excitations involved are collective, plasmonic type.

Probing Aromaticity of Borozene Through Optical and Dielectric Response: A Theoretical Study

In this work, we report electronic structure calculations aimed at computing the linear optical absorption spectrum and static dipole polarizablity of a newly proposed boron-based planar aromatic compound borozene (B12H6). For the purpose, we use the semiempirical INDO model Hamiltonian, accompanied by large-scale correlation calculations using the multi-reference singles-doubles configuration-interaction (MRSDCI) approach. We present detailed predictions about the energetics, polarization properties, and the nature of many-particle states contributing to various peaks in the linear absorption spectrum. Our results can be used to characterize this material in future optical absorption experiments. We also argue that one can deduce the aromaticity of the cluster from the optical absorption and static polarizability results.

A general purpose Fortran 90 electronic structure program for conjugated systems using Pariser–Parr–Pople model

Pariser–Parr–Pople (P-P-P) model Hamiltonian has been used extensively over the years to perform calculations of electronic structure and optical properties of π-conjugated systems successfully. In spite of tremendous successes of ab initio theory of electronic structure of large systems, the P-P-P model continues to be a popular one because of a recent resurgence in interest in the physics of π-conjugated polymers, fullerenes and other carbon-based materials. In this paper, we describe a Fortran 90 computer program developed by us, which uses P-P-P model Hamiltonian to not only solve Hartree–Fock (HF) equation for closed- and open-shell systems, but also for performing correlation calculations at the level of single configuration interactions (SCI) for molecular systems. Moreover, the code is capable of computing linear optical absorption spectrum at various levels, such as, tight-binding (TB) Hückel model, HF, SCI, and also of calculating the band structure using the Hückel model. The code also allows the user to solve the HF equation in the presence of finite external electric field, thus, permitting calculations of quantities such as static polarizabilities and electro-absorption spectra. Additionally, it can perform transformation of P-P-P model Hamiltonian from the atomic orbital (AO) representation (also called site representation) to the molecular orbital (MO) one, so that the transformed matrix elements can be used for high level post-HF calculations, such as, full CI (FCI), quadruple CI (QCI), and multi-reference singles–doubles CI (MRSDCI). We demonstrate the capabilities of our code by performing calculations of various properties on conjugated systems such as trans-polyacetylene ( t-PA), poly- para-phenylene (PPP), poly- para-phenylene-vinylene (PPV), oligo-acenes, and graphene nanodisks. Program summary Program title: ppp.x Catalogue identifier: AEFW_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEFW_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC license, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 79 900 No. of bytes in distributed program, including test data, etc.: 508 285 Distribution format: tar.gz Programming language: Fortran 90. Compilers used: Program has been tested with Intel Fortran Compiler (noncommercial version 11.1) and gfortran compiler (gcc version 4.4.0) with optimization option -O Computer: PCs, workstations Operating system: Linux. Code was developed and tested on various recent versions of Fedora including Fedora 11 (kernel version 2.6.29.4-167) Classification: 7.3, 16.1 External routines: This program needs to link with LAPACK/BLAS libraries compiled with the same compiler as the program. For the Intel Fortran Compiler we used the ACML library version 4.3.0, while for gfortran compiler we used the libraries supplied with the Fedora distribution. Nature of problem: The problem of interest at hand is the electronic structure of π-conjugated systems. For such systems, the effective π-electron P-P-P semi-empirical model Hamiltonian proposed by Pariser, Parr, and Pople offers an attractive alternative as compared to the ab initio approaches. The present program can solve the HF equations for both open- and closed-shell systems within the P-P-P model. Moreover, it can also include electron correlation effects at the singles CI level. Along with the wave functions and energies, various properties such as linear absorption spectra can also be computed. Solution method: The single-particle HF orbitals of a π-conjugated system are expressed as linear combinations of the p z -orbitals of individual atoms (assuming that the system is in the xy-plane). Then using the hopping and Coulomb parameters prescribed for the P-P-P method, the HF integro-differential equations are transformed into a matrix eigenvalue problem. Thereby, its solutions are obtained in a self-consistent manner, using the iterative diagonalizing technique. The HF orbitals thus obtained can be used to perform a variety of calculations such as the SCI, linear optical absorption spectrum, polarizability, electro-absorption spectrum, etc. Running time: The examples provided each only take a few seconds to run. For a large molecule or a polymer, however, the run time may be a few minutes.

Theoretical studies of UO2(OH)(H2O)n+, UO2(OH)2(H2O)n, NpO2(OH)(H2O)n, and PuO2(OH)(H2O)n+ (n≤21) complexes in aqueous solution

Extensive ab initio calculations have been carried out to study equilibrium structures, vibrational frequencies, and the nature of chemical bonds of hydrated UO(2)(OH)(+), UO(2)(OH)(2), NpO(2)(OH), and PuO(2)(OH)(+) complexes that contain up to 21 water molecules both in first and second hydration spheres in both aqueous solution and the gas phase. The structures have been further optimized by considering long-range solvent effects through a polarizable continuum dielectric model. The hydrolysis reaction Gibbs free energy of UO(2)(H(2)O)(5) (2+) is computed to be 8.11 kcal/mol at the MP2 level in good agreement with experiments. Our results reveal that it is necessary to include water molecules bound to the complex in the first hydration sphere for proper treatment of the hydrated complex and the dielectric cavity although water molecules in the second hydration sphere do not change the coordination complex. Structural reoptimization of the complex in a dielectric cavity seems inevitable to seek subtle structural variations in the solvent and to correlate with the observed spectra and thermodynamic properties in the aqueous environment. Our computations reveal dramatically different equilibrium structures in the gas phase and solution and also confirm the observed facile exchanges between the complex and bulk solvent. Complete active space multiconfiguration self-consistent field followed by multireference singles+doubles CI (MRSDCI) computations on smaller complexes confirm predominantly single-configurational nature of these species and the validity of B3LYP and MP2 techniques for these complexes in their ground states.

GEOMETRIES AND ENERGY SEPARATIONS OF ELECTRONIC STATES OF In3N, InN3, AND THEIR IONS

Spectroscopic properties of the low-lying electronic states of In 3 N , InN 3, and their ions are computed by the complete active-space self-consistent field (CASSCF) followed by multireference singles + doubles configuration interaction (MRSDCI) calculations. Our results predict that the spectra of In 3 N / InN 3 are substantially different from those of Ga 3 As / GaAs 3 and Al 3 P / AlP 3 tetramers. The ground state of In 3 N is a closed-shell 1 A ′1 state with a planar D 3h symmetry, whereas the ground state of InN 3 is a 1Σ+ state of linear In – N – N – N structure. The equilibrium geometries, vibrational frequencies, atomization energies, adiabatic ionization potentials, electron affinities, and other properties are discussed.

Large-scale correlated calculations of linear optical absorption and low-lying excited states of polyacenes: Pariser-Parr-Pople Hamiltonian

We present large-scale correlated calculations of the linear optical absorption spectrum of oligo-acenes containing up to seven benzene rings. For the calculations we used the Pariser-Parr-Pople (PPP) Hamiltonian, along with the configuration interaction (CI) technique at various levels such as the full CI, the quadruple CI, and multireference singles-doubles CI. The role of Coulomb parameters used in the PPP Hamiltonian was examined by considering standard Ohno parameters, as well as a screened set of parameters. A detailed analysis of the many-body character of the important excited states contributing to the linear absorption has also been performed. The results of our calculations have been compared extensively with the theoretical work of other authors, as well as with experiments.

Unusual geometries and spectroscopic properties of electronic states of In 2N 2

Twenty electronic states of In 2N 2 are computed using a complete active space multi-configuration self-consistent field followed by multi-reference singles + doubles configuration interaction (MRSDCI) computations. In contrast to other III–V tetramers, In 2N 2 exhibits nearly degenerate T-shaped triplet states and linear In–In–N–N 3Σ − state. The 1A g state with a rhombus structure was found to be 0.94 eV higher. Our computations show that anion photodetachment spectra of In 2N 2 could be substantially different from the spectra of Ga 2P 2 and related tetramers observed by Neumark and coworkers. The computed vibrational and Raman spectra are provided.

Electronic states and potential energy curves of InN 2, In 2N, and their ions

Potential energy curves of low-lying electronic states of InN 2, In 2N, and their cations and anions are investigated by the complete active space multi-configuration self-consistent field (CASSCF) followed by multi-reference singles + doubles configuration interaction (MRSDCI) computations. The neutral InN 2 and In 2N exhibit linear 2Π and 2 Σ u + ground states, respectively. The InNN + cation exhibits a linear C ∞v ground state whereas In 2N + ion exhibits two nearly-degenerate singlet and triplet ground states. Whereas In 2N − is strongly bound with an adiabatic electron affinity of 2.07 eV, the electron affinity of InNN is only 0.2 eV.

A correlated study of linear optical absorption in tetracene and pentacene

In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of tetracene and pentacene. For the purpose, we used Pariser–Parr–Pople (P–P–P) model Hamiltonian and quadruple-CI (QCI) and multi-reference single–double CI (MRSDCI) approaches. The role of Coulomb parameters used in P–P–P Hamiltonian was also examined by considering standard parameters as well as screened set of parameters. Results are presented both for the long- and the short-axis-polarized components of the spectra and wave functions of various excited states have also been analysed.

Electronic states and vibrational frequencies of the triatomic Ga 2N, GaN 2, and their cations and anions

The low-lying electronic states of Ga 2N, GaN 2 and the corresponding anions and cations have been investigated using B3LYP, MP2, CCD, complete active space multi-configuration self-consistent field technique (CASSCF) and multi-reference singles + doubles configuration interaction (MRSDCI) calculations. Potential energy surfaces, geometries, energy separations, adiabatic ionization energies, electron affinities, vibrational frequencies, and dissociation energies are computed.

Theory of two-photon absorption in poly(diphenyl) polyacetylenes

In this paper, we present a theoretical study of the nonlinear optical response of the newly discovered conjugated polymer poly(diphenyl)polyacetylene (PDPA). In particular, we compute the third-order nonlinear susceptibility corresponding to two-photon absorption process in PDPA using: (a) independent-particle Hückel model, and (b) using the correlated-electron Pariser-Parr-Pople (P-P-P) model coupled with various configuration-interaction methodologies such as the singles-configuration-interaction (SCI), the multi-reference-singles-doubles CI (MRSDCI), and the quadruples-CI (QCI) method. At all levels of theory, the polymer is found to exhibit highly anisotropic nonlinear optical response, distributed over two distinct energy scales. The low-energy response is predominantly polarized in the conjugation direction, and can be explained in terms of chain-based orbitals. The high-energy response of the polymer is found to be polarized perpendicular to the conjugation direction, and can be explained in terms of orbitals based on the side phenylene rings. Moreover, the intensity of the nonlinear optical response is also enhanced as compared to the corresponding polyenes, and can be understood in terms of reduced optical gap.