112 research outputs found
Deterministic construction of nodal surfaces within quantum Monte Carlo: the case of FeS
In diffusion Monte Carlo (DMC) methods, the nodes (or zeroes) of the trial
wave function dictate the magnitude of the fixed-node (FN) error. Within
standard DMC implementations, they emanate from short multideterminant
expansions, \textit{stochastically} optimized in the presence of a Jastrow
factor. Here, following a recent proposal, we follow an alternative route by
considering the nodes of selected configuration interaction (sCI) expansions
built with the CIPSI (Configuration Interaction using a Perturbative Selection
made Iteratively) algorithm. In contrast to standard implementations, these
nodes can be \textit{systematically} and \textit{deterministically} improved by
increasing the size of the sCI expansion. The present methodology is used to
investigate the properties of the transition metal sulfide molecule FeS. This
apparently simple molecule has been shown to be particularly challenging for
electronic structure theory methods due to the proximity of two low-energy
quintet electronic states of different spatial symmetry. In particular, we show
that, at the triple-zeta basis set level, all sCI results --- including those
extrapolated at the full CI (FCI) limit --- disagree with experiment, yielding
an electronic ground state of symmetry. Performing FN-DMC
simulation with sCI nodes, we show that the correct ground state
is obtained if sufficiently large expansions are used. Moreover, we show that
one can systematically get accurate potential energy surfaces and reproduce the
experimental dissociation energy as well as other spectroscopic constants.Comment: 8 pages, 2 figure and 4 table
Hybrid stochastic-deterministic calculation of the second-order perturbative contribution of multireference perturbation theory
A hybrid stochastic-deterministic approach for computing the second-order
perturbative contribution within multireference perturbation theory
(MRPT) is presented. The idea at the heart of our hybrid scheme --- based on a
reformulation of as a sum of elementary contributions associated with
each determinant of the MR wave function --- is to split into a
stochastic and a deterministic part. During the simulation, the stochastic part
is gradually reduced by dynamically increasing the deterministic part until one
reaches the desired accuracy. In sharp contrast with a purely stochastic MC
scheme where the error decreases indefinitely as (where is the
computational time), the statistical error in our hybrid algorithm displays a
polynomial decay with in the examples considered here. If
desired, the calculation can be carried on until the stochastic part entirely
vanishes. In that case, the exact result is obtained with no error bar and no
noticeable computational overhead compared to the fully-deterministic
calculation. The method is illustrated on the F and Cr molecules. Even
for the largest case corresponding to the Cr molecule treated with the
cc-pVQZ basis set, very accurate results are obtained for for an
active space of (28e,176o) and a MR wave function including up to determinants.Comment: 8 pages, 5 figure
Spin adaptation with determinant-based selected configuration interaction
Selected configuration interaction (sCI) methods, when complemented with a
second order perturbative correction, provide near full configuration
interaction (FCI) quality energies with only a small fraction of the Slater
determinants of the FCI space. The selection of the determinants is often
implemented in a determinant-based formalism, and therefore does not provide
spin adapted wave functions. In other words, sCI wave functions are not
eigenfunctions of the operator. In some situations, having a
spin adapted wave function is essential for the proper convergence of the
method. We propose an efficient algorithm which, given an arbitrary determinant
space, generates all the missing Slater determinants allowing one to obtain
spin adapted wave functions while avoiding working with configuration state
functions. For example, generating all the possible determinants with 6 up-spin
and 6 down-spin electrons in 12 open shells takes 21 CPU cycles per generated
Slater determinant. We also propose a modification of the denominators in the
Epstein-Nesbet perturbation theory reducing significantly the non-invariance of
the second order correction with respect to different values of the spin
quantum number . The computational cost of this correction is also
negligible
Excitation energies from diffusion Monte Carlo using selected Configuration Interaction nodes
Quantum Monte Carlo (QMC) is a stochastic method which has been particularly
successful for ground-state electronic structure calculations but mostly
unexplored for the computation of excited-state energies. Here, we show that,
within a Jastrow-free QMC protocol relying on a deterministic and systematic
construction of nodal surfaces using selected configuration interaction (sCI)
expansions, one is able to obtain accurate excitation energies at the
fixed-node diffusion Monte Carlo (FN-DMC) level. This evidences that the
fixed-node errors in the ground and excited states obtained with sCI wave
functions cancel out to a large extent. Our procedure is tested on two small
organic molecules (water and formaldehyde) for which we report all-electron
FN-DMC calculations. For both the singlet and triplet manifolds, accurate
vertical excitation energies are obtained with relatively compact
multideterminant expansions built with small (typically double-) basis
sets.Comment: 8 pages, 3 figure
A Density-Based Basis-Set Incompleteness Correction for GW Methods
9 pages, 2 figures (supporting information available)International audienceSimilar to other electron correlation methods, many-body perturbation theory methods based on Green functions, such as the so-called approximation, suffer from the usual slow convergence of energetic properties with respect to the size of the one-electron basis set. This displeasing feature is due to lack of explicit electron-electron terms modeling the infamous Kato electron-electron cusp and the correlation Coulomb hole around it. Here, we propose a computationally efficient density-based basis set correction based on short-range correlation density functionals which significantly speeds up the convergence of energetics towards the complete basis set limit. The performance of this density-based correction is illustrated by computing the ionization potentials of the twenty smallest atoms and molecules of the GW100 test set at the perturbative (or ) level using increasingly large basis sets. We also compute the ionization potentials of the five canonical nucleobases (adenine, cytosine, thymine, guanine, and uracil) and show that, here again, a significant improvement is obtained
Excited States From State Specific Orbital Optimized Pair Coupled Cluster
The pair coupled cluster doubles (pCCD) method (where the excitation manifold
is restricted to electron pairs) has a series of interesting features. Among
others, it provides ground-state energies very close to what is obtained with
doubly-occupied configuration interaction (DOCI), but with polynomial cost
(compared with the exponential cost of the latter). Here, we address whether
this similarity holds for excited states, by exploring the symmetric
dissociation of the linear \ce{H4} molecule. When ground-state Hartree-Fock
(HF) orbitals are employed, pCCD and DOCI excited-state energies do not match,
a feature that is assigned to the poor HF reference. In contrast, by optimizing
the orbitals at the pCCD level (oo-pCCD) specifically for each excited state,
the discrepancies between pCCD and DOCI decrease by one or two orders of
magnitude. Therefore, the pCCD and DOCI methodologies still provide comparable
energies for excited states, but only if suitable, state-specific orbitals are
adopted. We also assessed whether a pCCD approach could be used to directly
target doubly-excited states, without having to resort to the
equation-of-motion (EOM) formalism. In our oo-pCCD model, excitation
energies were extracted from the energy difference between separate oo-pCCD
calculations for the ground state and the targeted excited state. For a set
comprising the doubly-excited states of \ce{CH+}, \ce{BH}, nitroxyl,
nitrosomethane, and formaldehyde, we found that oo-pCCD provides quite
accurate excitation energies, with root mean square deviations (with respect to
full configuration interaction results) lower than CC3 and comparable to
EOM-CCSDT, two methods with much higher computational cost.Comment: 12 pages, 4 figure
Ground- and Excited-State Dipole Moments and Oscillator Strengths of Full Configuration Interaction Quality
We report ground- and excited-state dipole moments and oscillator strengths
(computed in different ``gauges'' or representations) of full configuration
interaction (FCI) quality using the selected configuration interaction method
known as \textit{Configuration Interaction using a Perturbative Selection made
Iteratively} (CIPSI). Thanks to a set encompassing 35 ground- and excited-state
properties computed in 11 small molecules, the present near-FCI estimates allow
us to assess the accuracy of high-order coupled-cluster (CC) calculations
including up to quadruple excitations. In particular, we show that incrementing
the excitation degree of the CC expansion (from CCSD to CCSDT or from CCSDT to
CCSDTQ) reduces the average error with respect to the near-FCI reference values
by approximately one order of magnitude.Comment: 14 pages, 8 figures (supporting information available
Maritime Transportation: Let\u27s Slow Down a Bit
Maritime transportation is a major contributor to the world economy, but has significant social and environmental impacts. Each impact calls for different technical or operational solutions. Amongst these solutions, we found that speed reduction measures appear to mitigate several issues: (1) collision with wildlife; (2) collision with non-living objects; (3) underwater noise; (4) invasive species; and (5) gas emission. We do not pretend that speed reduction is the best solution for each individual issue mentioned in this paper, but we argue that it could be a key solution to significantly reduce these threats all together. Further interdisciplinary research is required to balance private economic costs of speed reduction measures with environmental and social benefits emerging from all mitigated issues
Addressing marine and coastal governance conflicts at the interface of multiple sectors and jurisdictions
Marine and coastal activities are closely interrelated, and conflicts among different sectors can undermine management and conservation objectives. Governance systems for fisheries, power generation, irrigation, aquaculture, marine biodiversity conservation, and other coastal and maritime activities are typically organized to manage conflicts within sectors, rather than across them. Based on the discussions around eight case studies presented at a workshop held in Brest in June 2019, this paper explores institutional approaches to move beyond managing conflicts within a sector. We primarily focus on cases where the groups and sectors involved are heterogeneous in terms of: the jurisdiction they fall under; their objectives; and the way they value ecosystem services. The paper first presents a synthesis of frameworks for understanding and managing cross-sectoral governance conflicts, drawing from social and natural sciences. We highlight commonalities but also conceptual differences across disciplines to address these issues. We then propose a novel analytical framework which we used to evaluate the eight case studies. Based on the main lessons learned from case studies, we then discuss the feasibility and key determinants of stakeholder collaboration as well as compensation and incentive schemes. The discussion concludes with future research needs to support policy development and inform integrated institutional regimes that consider the diversity of stakeholder interests and the potential benefits of cross-sectoral coordination
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