1,251 research outputs found
Pair extended coupled cluster doubles
The accurate and efficient description of strongly correlated systems remains
an important challenge for computational methods. Doubly occupied configuration
interaction (DOCI), in which all electrons are paired and no correlations which
break these pairs are permitted, can in many cases provide an accurate account
of strong correlations, albeit at combinatorial computational cost. Recently,
there has been significant interest in a method we refer to as pair coupled
cluster doubles (pCCD), a variant of coupled cluster doubles in which the
electrons are paired. This is simply because pCCD provides energies nearly
identical to those of DOCI, but at mean-field computational cost (disregarding
the cost of the two-electron integral transformation). Here, we introduce the
more complete pair extended coupled cluster doubles (pECCD) approach which,
like pCCD, has mean-field cost and reproduces DOCI energetically. We show that
unlike pCCD, pECCD also reproduces the DOCI wave function with high accuracy.
Moreoever, pECCD yields sensible albeit inexact results even for attractive
interactions where pCCD breaks down.Comment: submitted manuscrip
Polynomial Similarity Transformation Theory: A smooth interpolation between coupled cluster doubles and projected BCS applied to the reduced BCS Hamiltonian
We present a similarity transformation theory based on a polynomial form of a
particle-hole pair excitation operator. In the weakly correlated limit, this
polynomial becomes an exponential, leading to coupled cluster doubles. In the
opposite strongly correlated limit, the polynomial becomes an extended Bessel
expansion and yields the projected BCS wavefunction. In between, we interpolate
using a single parameter. The effective Hamiltonian is non-hermitian and this
Polynomial Similarity Transformation Theory follows the philosophy of
traditional coupled cluster, left projecting the transformed Hamiltonian onto
subspaces of the Hilbert space in which the wave function variance is forced to
be zero. Similarly, the interpolation parameter is obtained through minimizing
the next residual in the projective hierarchy. We rationalize and demonstrate
how and why coupled cluster doubles is ill suited to the strongly correlated
limit whereas the Bessel expansion remains well behaved. The model provides
accurate wave functions with energy errors that in its best variant are smaller
than 1\% across all interaction stengths. The numerical cost is polynomial in
system size and the theory can be straightforwardly applied to any realistic
Hamiltonian
Actinide chemistry using singlet-paired coupled cluster and its combinations with density functionals
Singlet-paired coupled cluster doubles (CCD0) is a simplification of CCD that
relinquishes a fraction of dynamic correlation in order to be able to describe
static correlation. Combinations of CCD0 with density functionals that recover
specifically the dynamic correlation missing in the former have also been
developed recently. Here, we assess the accuracy of CCD0 and CCD0+DFT (and
variants of these using Brueckner orbitals) as compared to well-established
quantum chemical methods for describing ground-state properties of singlet
actinide molecules. The actinyl series (UO, NpO,
PuO), the isoelectronic NUN, and Thorium (ThO, ThO) and
Nobelium (NoO, NoO) oxides are studied.Comment: 8 page
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
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