314 research outputs found
Low-temperature chemistry using the R-matrix method
Techniques for producing cold and ultracold molecules are enabling the study
of chemical reactions and scattering at the quantum scattering limit, with only
a few partial waves contributing to the incident channel, leading to the
observation and even full control of state-to-state collisions in this regime.
A new R-matrix formalism is presented for tackling problems involving low- and
ultra-low energy collisions. This general formalism is particularly appropriate
for slow collisions occurring on potential energy surfaces with deep wells. The
many resonance states make such systems hard to treat theoretically but offer
the best prospects for novel physics: resonances are already being widely used
to control diatomic systems and should provide the route to steering ultracold
reactions. Our R-matrix-based formalism builds on the progress made in
variational calculations of molecular spectra by using these methods to provide
wavefunctions for the whole system at short internuclear distances, (a regime
known as the inner region). These wavefunctions are used to construct collision
energy-dependent R-matrices which can then be propagated to give cross sections
at each collision energy. The method is formulated for ultracold collision
systems with differing numbers of atoms.Comment: Presented at Faraday Discussion on the Theory of Chemical Reactions
Published in Faraday Discussion
ExoMol line lists XVIII. The high temperature spectrum of VO
An accurate line list, VOMYT, of spectroscopic transitions is presented for
hot VO. The 13 lowest electronic states are considered. Curves and couplings
are based on initial {\it ab initio} electronic structure calculations and then
tuned using available experimental data. Dipole moment curves, used to obtain
transition intensities, are computed using high levels of theory (e.g.
MRCI/aug-cc-pVQZ using state-specific or minimal-state CAS for dipole moments).
This line list contains over 277 million transitions between almost 640,000
energy levels. It covers the wavelengths longer than 0.29 m and includes
all transitions from energy levels within the lowest nine electronic states
which have energies less than 20,000 \cm{} to upper states within the lowest 13
electronic states which have energies below 50,000 \cm{}. The line lists give
significantly increased absorption at infrared wavelengths compared to
currently available VO line lists. The full line lists is made available in
electronic form via the CDS database and at www.exomol.com.Comment: MNRAS in pres
Ab initio calculations to support accurate modelling of the rovibronic spectroscopy calculations of vanadium monoxide (VO)
Accurate knowledge of the rovibronic near-infrared and visible spectra of
vanadium monoxide (VO) is very important for studies of cool stellar and hot
planetary atmospheres. Here, the required ab initio dipole moment and
spin-orbit coupling curves for VO are produced. This data forms the basis of a
new VO line list considering 13 different electronic states and containing over
277 million transitions. Open shell transition, metal diatomics are challenging
species to model through ab initio quantum mechanics due to the large number of
low-lying electronic states, significant spin-orbit coupling and strong static
and dynamic electron correlation. Multi-reference configuration interaction
methodologies using orbitals from a complete active space self-consistent-field
(CASSCF) calculation are the standard technique for these systems. We use
different state-specific or minimal-state CASSCF orbitals for each electronic
state to maximise the calculation accuracy. The off-diagonal dipole moment
controls the intensity of electronic transitions. We test finite-field
off-diagonal dipole moments, but found that (1) the accuracy of the excitation
energies were not sufficient to allow accurate dipole moments to be evaluated
and (2) computer time requirements for perpendicular transitions were
prohibitive. The best off-diagonal dipole moments are calculated using
wavefunctions with different CASSCF orbitals.Comment: Molecular Physics, 201
SciX: Scalable and sustainable authentic research experiences for high-school students
In NSW, the new Year 12 Higher School Certificate Science Extension course recommends students find university mentorship to support their individual research projects. The SciX high-school outreach program (unsw.to/scix) has been developed and refined to meet this demand in an equitable, sustainable, scalable, effective and quality-controlled way.
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SciX centres around an intensive one-week authentic research experience with online pre-work and post-summer-school Q&A sessions. High school students select a research area and are placed in small groups led by SciX mentors, usually paid PhD researchers. Students are taught disciplinary research topics and tools, and then supported to develop their individual hypothesis and conduct their research. Qualitative and quantitative surveys show that students really enjoy the experience – especially their interaction with mentors, increase their self-identification as a scientist and develop crucial transferable and scientific skills.
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Through their role as SciX mentors, paid PhD students are supported in developing important professional skills, e.g., in supervising, mentoring, teaching and management. Project development and delivery is carefully scaffolded with training, structured support and regular reviews. Time expectations are clearly set and reasonable to avoid interfering with PhD progression.
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The success of the program is clearly demonstrated by its strong increasing enrolments, now exceeding 150 students annually. The program is clearly addressing equity, diversity and inclusion goals, with our 2023 enrolments 63% female, 40% fee-waiver positions supporting students from a low socioeconomic, regional or rural area.Â
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This talk will be targeted at those interested in supporting research for high-school and/or undergraduate (pre-Honours) research students through supervision and/or program design. As inspiration for how you might deliver this enriching student experience, I will describe how SciX has addressed key challenges, specifically careful project design, program design appropriate to the local context, securing funding and developing a scalable team structure
Efficient calculation of integrals in mixed ramp-Gaussian basis sets
Algorithms for the efficient calculation of two-electron integrals in the newly developed mixed ramp-Gaussian basis sets are presented, alongside a Fortran90 implementation of these algorithms, RampItUp.These new basis sets have significant potential to (1) give some speed-up (estimated at up to 20% for large molecules in fully optimised code) to general-purpose Hartree-Fock (HF) and density functional theory quantum chemistry calculations, replacing all-Gaussian basis sets, and (2) give very large speed-ups for calculations of core-dependent properties, such as electron density at the nucleus, NMR parameters, relativistic corrections, and total energies, replacing the current use of Slater basis functions or very large specialised all-Gaussian basis sets for these purposes. This initial implementation already demonstrates roughly 10% speed-ups in HF/R-31G calculations compared to HF/6-31G calculations for large linear molecules, demonstrating the promise of this methodology, particularly for the second application. As well as the reduction in the total primitive number in R-31G compared to 6-31G, this timing advantage can be attributed to the significant reduction in the number of mathematically complex intermediate integrals after modelling each ramp-Gaussian basis-function-pair as a sum of ramps on a single atomic centre
Low temperature scattering with the R-matrix method: argon-argon scattering
Results for elastic atom-atom scattering are obtained as a first practical
application of RmatReact, a new code for generating high-accuracy scattering
observables from potential energy curves. RmatReact has been created in
response to new experimental methods which have paved the way for the routine
production of ultracold atoms and molecules, and hence the experimental study
of chemical reactions involving only a small number of partial waves. Elastic
scattering between argon atoms is studied here. There is an unresolved
discrepancy between different argon-argon potential energy curves which give
different numbers of vibrational bound states and different scattering lengths
for the argon-argon dimer. Depending on the number of bound states, the
scattering length is either large and positive or large and negative.
Scattering observables, specifically the scattering length, effective range,
and partial and total cross-sections, are computed at low collision energies
and compared to previous results. In general, good agreement is obtained,
although our full scattering treatment yields resonances which are slightly
lower in energy and narrower than previous determinations using the same
potential energy curve.Comment: 26 pages, 9 figures, 3 table
The {\it ab initio} calculation of spectra of open shell diatomic molecules
The spectra (rotational, rotation-vibrational or electronic) of diatomic
molecules due to transitions involving only closed-shell ()
electronic states follow very regular, simple patterns and their theoretical
analysis is usually straightforward. On the other hand, open-shell electronic
states lead to more complicated spectral patterns and, moreover, often appear
as a manifold of closely lying electronic states, leading to perturbations with
even larger complexity. This is especially true when at least one of the atoms
is a transition metal. Traditionally these complex cases have been analysed
using approaches based on perturbation theory, with semi-empirical parameters
determined by fitting to spectral data.
Recently the needs of two rather diverse scientific areas have driven the
demand for improved theoretical models of open-shell diatomic systems based on
an \emph{ab initio} approach, these areas are ultracold chemistry and the
astrophysics of "cool" stars, brown dwarfs and most recently extrasolar
planets. However, the complex electronic structure of these molecules combined
with the accuracy requirements of high-resolution spectroscopy render such an
approach particularly challenging. This review describes recent progress in
developing methods for directly solving the effective Schr\"odinger equation
for open-shell diatomic molecules, with a focus on molecules containing a
transtion metal. It considers four aspects of the problem: 1. The electronic
structure problem, 2. Non-perturbative treatments of the curve couplings, 3.
The solution of the nuclear motion Schr\"odinger equation, 4. The generation of
accurate electric dipole transition intensities. Examples of applications are
used to illustrate these issues.Comment: Topical Revie
Python for chemists: a problem-orientated introduction to scientific programming
Programming is an essential skill in modern science, yet it is not routinely or systematically taught as part of most undergraduate science courses. Many students pick up an outside interest in programming, but those who do not may be left behind, and lose access to an essential part of the modern scientist’s toolbox. A compulsory programming module for all first-year science students is one possible solution, but such a general education may prove remote from specific disciplinary needs. The most useful skills for non-specialists using programming in their research or work are different from those needed by specialist computer scientists, with more emphasis on data generation, processing, exploration, analysis, and visualisation.
Within the University of New South Wales School of Chemistry, we have designed a Python in Chemistry Honours module for final-year undergraduates and research students, designed to directly tackle these challenges and offer an alternative to, or complement, earlier structured programming training. There are three main learning activities supported by class discussions, workshops, and explicit incorporation of meta-cognition and communication within assessment.
Self-paced online modules, self-selected with beginning and advanced modules to support diverse student programming backgrounds;
Discipline-specific challenges as assignments;
A capstone major project designed by the student usually to support their disciplinary research
Low-temperature chemistry using the R-matrix method
Techniques for producing cold and ultracold molecules are enabling the study of chemical reactions and scattering at the quantum scattering limit, with only a few partial waves contributing to the incident channel, leading to the observation and even full control of state-to-state collisions in this regime. A new R-matrix formalism is presented for tackling problems involving low- and ultra-low energy collisions. This general formalism is particularly appropriate for slow collisions occurring on potential energy surfaces with deep wells. The many resonance states make such systems hard to treat theoretically but offer the best prospects for novel physics: resonances are already being widely used to control diatomic systems and should provide the route to steering ultracold reactions. Our R-matrix-based formalism builds on the progress made in variational calculations of molecular spectra by using these methods to provide wavefunctions for the whole system at short internuclear distances, (a regime known as the inner region). These wavefunctions are used to construct collision energy-dependent R-matrices which can then be propagated to give cross sections at each collision energy. The method is formulated for ultracold collision systems with differing numbers of atoms
General Mathematical Formulation of Scattering Processes in Atom-Diatomic Collisions in the RmatReact Methodology
Accurately modelling cold and ultracold reactive collisions occuring over
deep potential wells, such as \ce{D+ + H2 -> H+ + HD}, requires the development
of new theoretical and computational methodologies. One potentially useful
framework is the R-matrix method adopted widely for electron-molecule
collisions which has more recently been applied to non-reactive heavy particle
collisions such as Ar-Ar. The existing treatment of non-reactive elastic and
inelastic scattering needs to be substantially extended to enable modelling of
reactive collisions: this is the subject of this paper. Herein, we develop the
general mathematical formulation for non-reactive elastic and inelastic
scattering, photo-association, photo-dissociation, charge exchange and reactive
scattering using the R-matrix method. Of particular note is that the inner
region, of central importance to calculable R-matrix methodologies, must be
finite in all scattering coordinates rather than a single scattering coordinate
as for non-reactive scattering. % The choice of coordinate set and basis
function is these cases becomes more complexThis introduces substantial
challenges to the basis sets utilised in practical calculations as integrals
over finite domains are often much more challenging than over infinite domains
for this problem.Comment: Submitted as part of the issue of Phil. Trans. Roy. Soc. A special
issue on "Advances in hydrogen molecular ions: H3+, H5+ and beyond
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