2 research outputs found
Massively Parallel Implementation of Explicitly Correlated Coupled-Cluster Singles and Doubles Using TiledArray Framework
A new
distributed-memory massively parallel implementation of standard
and explicitly correlated (F12) coupled-cluster singles and doubles
(CCSD) with canonical <i>O</i>(<i>N</i><sup>6</sup>) computational complexity is described. The implementation is based
on the TiledArray tensor framework. Novel features of the implementation
include (a) all data greater than <i>O</i>(<i>N</i>) is distributed in memory and (b) the mixed use of density fitting
and integral-driven formulations that optionally allows to avoid storage
of tensors with three and four unoccupied indices. Excellent strong
scaling is demonstrated on a multicore shared-memory computer, a commodity
distributed-memory computer, and a national-scale supercomputer. The
performance on a shared-memory computer is competitive with the popular
CCSD implementations in ORCA and Psi4. Moreover, the CCSD performance
on a commodity-size cluster significantly improves on the state-of-the-art
package NWChem. The large-scale parallel explicitly correlated coupled-cluster
implementation makes routine accurate estimation of the coupled-cluster
basis set limit for molecules with 20 or more atoms. Thus, it can
provide valuable benchmarks for the merging reduced-scaling coupled-cluster
approaches. The new implementation allowed us to revisit the basis
set limit for the CCSD contribution to the binding energy of π-stacked
uracil dimer, a challenging paradigm of π-stacking interactions
from the S66 benchmark database. The revised value for the CCSD correlation
binding energy obtained with the help of quadruple-ζ CCSD computations,
−8.30 ± 0.02 kcal/mol, is significantly different from
the S66 reference value, −8.50 kcal/mol, as well as other CBS
limit estimates in the recent literature
Toward Accurate Post-Born–Oppenheimer Molecular Simulations on Quantum Computers: An Adaptive Variational Eigensolver with Nuclear-Electronic Frozen Natural Orbitals
Nuclear
quantum effects such as zero-point energy and
hydrogen
tunneling play a central role in many biological and chemical processes.
The nuclear-electronic orbital (NEO) approach captures these effects
by treating selected nuclei quantum mechanically on the same footing
as electrons. On classical computers, the resources required for an
exact solution of NEO-based models grow exponentially with system
size. By contrast, quantum computers offer a means of solving this
problem with polynomial scaling. However, due to the limitations of
current quantum devices, NEO simulations are confined to the smallest
systems described by minimal basis sets, whereas realistic simulations
beyond the Born–Oppenheimer approximation require more sophisticated
basis sets. For this purpose, we herein extend a hardware-efficient
ADAPT-VQE method to the NEO framework in the frozen natural orbital
(FNO) basis. We demonstrate on H2 and D2 molecules
that the NEO-FNO-ADAPT-VQE method reduces the CNOT count by several
orders of magnitude relative to the NEO unitary coupled cluster method
with singles and doubles while maintaining the desired accuracy. This
extreme reduction in the CNOT gate count is sufficient to permit practical
computations employing the NEO methodan important step toward
accurate simulations involving nonclassical nuclei and non-Born–Oppenheimer
effects on near-term quantum devices. We further show that the method
can capture isotope effects, and we demonstrate that inclusion of
correlation energy systematically improves the prediction of difference
in the zero-point energy (ΔZPE) between isotopes