8 research outputs found
New Approaches for ab initio Calculations of Molecules with Strong Electron Correlation
Reliable quantum chemical methods for the description of molecules with
dense-lying frontier orbitals are needed in the context of many chemical
compounds and reactions. Here, we review developments that led to our
newcomputational toolbo x which implements the quantum chemical density matrix
renormalization group in a second-generation algorithm. We present an overview
of the different components of this toolbox.Comment: 19 pages, 1 tabl
A Quantum Computing Implementation of Nuclear-Electronic Orbital (NEO) Theory: Towards an Exact pre-Born-Oppenheimer Formulation of Molecular Quantum Systems
Nuclear quantum phenomena beyond the Born-Oppenheimer approximation are known
to play an important role in a growing number of chemical and biological
processes. While there exists no unique consensus on a rigorous and efficient
implementation of coupled electron-nuclear quantum dynamics, it is recognised
that these problems scale exponentially with system size on classical
processors and therefore may benefit from quantum computing implementations.
Here, we introduce a methodology for the efficient quantum treatment of the
electron-nuclear problem on near-term quantum computers, based upon the
Nuclear-Electronic Orbital (NEO) approach. We generalize the electronic
two-qubit tapering scheme to include nuclei by exploiting symmetries inherent
in the NEO framework; thereby reducing the hamiltonian dimension, number of
qubits, gates, and measurements needed for calculations. We also develop
parameter transfer and initialisation techniques, which improve convergence
behavior relative to conventional initialisation. These techniques are applied
to H and malonaldehyde for which results agree with Nuclear-Electronic
Orbital Full Configuration Interaction and Nuclear-Electronic Orbital Complete
Active Space Configuration Interaction benchmarks for ground state energy to
within Ha and entanglement entropy to within . These
implementations therefore significantly reduce resource requirements for full
quantum simulations of molecules on near-term quantum devices while maintaining
high accuracy.Comment: 26 pages, 7 figures, 10 table
Analytical gradients for excitation energies from frozen-density embedding
The formulation of analytical excitation-energy gradients from time-dependent density functional theory within the frozen-density embedding framework is presented. In addition to a comprehensive mathematical derivation, we discuss details of the numerical implementation in the Slater-function based Amsterdam Density Functional (ADF) program. Particular emphasis is put on the consistency in the use of approximations for the evaluation of second- and third-order non-additive kinetic-energy and exchangeâcorrelation functional derivatives appearing in the final expression for the excitation-energy gradient. We test the implementation for different chemical systems in which molecular excited-state potential-energy curves are affected by another subsystem. It is demonstrated that the analytical implementation for the evaluation of excitation-energy gradients yields results in close agreement with data from numerical differentiation. In addition, we show that our analytical results are numerically more stable and thus preferable over the numerical ones.ISSN:1463-9084ISSN:1463-907
Nonadiabatic Nuclear-Electron Dynamics: A Quantum Computing Approach
Coupled quantum electron-nuclear dynamics is oftenassociatedwith the Born-Huang expansion of the molecular wave functionand the appearance of nonadiabatic effects as a perturbation. On theother hand, native multicomponent representations of electrons andnuclei also exist, which do not rely on any a priori approximation.However, their implementation is hampered by prohibitive scaling.Consequently, quantum computers offer a unique opportunity for extendingtheir use to larger systems. Here, we propose a quantum algorithmfor simulating the time-evolution of molecular systems and apply itto proton transfer dynamics in malonaldehyde, described as a rigidscaffold. The proposed quantum algorithm can be easily generalizedto include the explicit dynamics of the classically described molecularscaffold. We show how entanglement between electronic and nucleardegrees of freedom can persist over long times if electrons do notfollow the nuclear displacement adiabatically. The proposed quantumalgorithm may become a valid candidate for the study of such phenomenawhen sufficiently powerful quantum computers become available
Toward Accurate Post-BornâOppenheimer 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 BornâOppenheimer 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-BornâOppenheimer
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 (ÎZPE) between isotopes