22 research outputs found
A Fortran 90 Hartree-Fock program for one-dimensional periodic -conjugated systems using Pariser-Parr-Pople model
Pariser-Parr-Pople (P-P-P) model Hamiltonian is employed frequently to study
the electronic structure and optical properties of -conjugated systems. In
this paper we describe a Fortran 90 computer program which uses the P-P-P model
Hamiltonian to solve the Hartree-Fock (HF) equation for infinitely long,
one-dimensional, periodic, -electron systems. The code is capable of
computing the band structure, as also the linear optical absorption spectrum,
by using the tight-binding (TB) and the HF methods. Furthermore, using our
program the user can solve the HF equation in the presence of a finite external
electric field, thereby, allowing the simulation of gated systems. We apply our
code to compute various properties of polymers such as -polyacetylene
(-PA), poly-\emph{para}-phenylene (PPP), and armchair and zigzag graphene
nanoribbons, in the infinite length limit.Comment: 33 pages, 11 figures (included), submitted for publicatio
The possible correlation of carcinogenic activity with electronic structure of benz(a)anthracene
The electronic structure of benz(a)anthracene based on the sigma and pi electrons was predicted by the modified intermediate neglect of differential overlap (MINDO) molecular orbital method and compared with the pi electronic structure determined by the Pople method. The crystalline molecular structure was used for both methods. The Pople calculation was also done on the aromatic molecular structure and a combination structure which assumed the bond lengths of the crystal structure and the bond angles of the aromatic structure. Chemical properties predicted by the MINDO and POPLE electronic structures were compared; the MINDO results provided the best agreement with experimental results. Based on the MINDO results, a bonding model for benz(a)anthracene was proposed and was found to be consistent with the known chemical reactivity of benz(a)anthracene. The carcinogenic activity of benz(a)anthracene was considered and possible general types of interactions between the molecule and cellular proteins or nucleic acids was suggested. Several suggestions for additional study were made --Abstract, page iii
Pariser-Parr-Pople Model based Investigation of Ground and Low-Lying Excited States of Long Acenes
Several years back Angliker et al [Chem. Phys. Lett. 1982, 87, 208] predicted
nonacene to be the first linear acene with the triplet state as
the ground state, instead of the singlet state. However, contrary
to that prediction, in a recent experimental work T\"onshoff 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.
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. 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 wavefunction of the correlated singlet ground state and find
that the longer acenes exhibit tendency towards a 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.Comment: 58 pages (including supplementary information), 12 figures (included
Tunable Optoelectronic Properties of Triply-Bonded Carbon Molecules with Linear and Graphyne Substructures
In this paper we present a detailed computational study of the electronic
structure and optical properties of triply-bonded hydrocarbons with linear, and
graphyne substructures, with the aim of identifying their potential in
opto-electronic device applications. For the purpose, we employed a correlated
electron methodology based upon the Pariser-Parr-Pople model Hamiltonian,
coupled with the configuration interaction (CI) approach, and studied
structures containing up to 42 carbon atoms. Our calculations, based upon
large-scale CI expansions, reveal that the linear structures have intense
optical absorption at the HOMO-LUMO gap, while the graphyne ones have those at
higher energies. Thus, the opto-electronic properties depend on the topology of
the {graphyne substructures, suggesting that they can be tuned by means of
structural modifications. Our results are in very good agreement with the
available experimental data.Comment: main text 29 pages + 4 figures + 1 TOC graphic (included), supporting
information 21 page
Efficient computation of the second-Born self-energy using tensor-contraction operations
In the nonequilibrium Green's function approach, the approximation of the
correlation self-energy at the second-Born level is of particular interest,
since it allows for a maximal speed-up in computational scaling when used
together with the Generalized Kadanoff-Baym Ansatz for the Green's function.
The present day numerical time-propagation algorithms for the Green's function
are able to tackle first principles simulations of atoms and molecules, but
they are limited to relatively small systems due to unfavourable scaling of
self-energy diagrams with respect to the basis size. We propose an efficient
computation of the self-energy diagrams by using tensor-contraction operations
to transform the internal summations into functions of external low-level
linear algebra libraries. We discuss the achieved computational speed-up in
transient electron dynamics in selected molecular systems.Comment: 9 pages, 4 figures, 1 tabl
Spectroscopic properties of conjugated systems
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Electrochemically-modulated liquid chromatography (EMLC): Column design, retention processes, and applications
This dissertation explores a new separation technique, electrochemically modulated liquid chromatography (EMLC), from the column design, retention processes, to the pharmaceutical applications. A literature review, general summary, and perspectives of this technique are also described. Chapter 1 presents the newly designed EMLC column. The principal modification of this design is to connect the porous stainless steel column as counter electrode as opposed to part of working electrode in the previous design. The improvement in performance from this modification results in a shorter response time to changes in applied potential (E appl) and a better control of E appl at cathodic values of E appl. The performance of the new design is presented and compared to the previous design;Chapter 2 describes the study of retention processes of analytes on EMLC. A mixture of substituted aromatic compounds has been investigated to examine the influence of E appl to retention. Results show that donor-acceptor interactions dominate the retention processes and that the analytes with larger submolecular polarity parameters or higher energy levels of highest occupied molecular orbital display larger sensitivities in retention to changes in E appl.;In Chapter 3, EMLC has been applied to the separation of a mixture of structurally similar corticosteroids. Changes in the E appl to the column markedly affected the efficiency as well as the elution order of the separation, with the mixture fully resolved at large negative values of E appl. Mechanistic aspects in terms of the influence of changes in the E appl on the extent of the interactions between these analytes and the stationary phase are briefly discussed;In Chapter 4, the separation of a mixture of benzodiazepines has been investigated by EMLC. Changes in the E appl to the stationary phase strongly alter the retention of all analytes. The observed dependencies of retention have the unusual effect of stretching both ends of the chromatogram as E appl becomes more negative. That is, the retention for some of the benzodiazepines increases as E appl moves negatively, whereas that for some of the other benzodiazepines decreases. The combined weight of these dependencies results in the ability to achieve a fully resolved separation of the mixture while only marginally increasing the overall elution time