15

It is hard to claim that any FCI code overcomes the exponential wall, especially for strongly correlated systems. There are many algorithms, e.g. CDFCI, HCI, FCIQMC, ACI, etc., which significantly reduce the computational cost of direct FCI calculations and represent wavefunctions by sparse vectors. However, in my opinion, all of them only reduce the ...


8

Not all determinants contribute significant correlation, especially configurations for excitations of multiple electrons to higher energy orbitals, which is the basis for CAS methods; i.e. only choose the “important” orbitals in which to perform FCI. Doing so works acceptably for weakly correlated systems, but since FCI is still performed within the active ...


7

To start with, we do have: LUCIA, a Correlation Program, 2019, developed by Kähler and Olsen. Nonorthogonal internally contracted multi-configurational perturbation theory and Dynamic correlation for nonorthogonal reference states. Q-CHEM. GronOR: Massively parallel and GPU- accelerated non-orthogonal configuration interaction for large molecular systems. ...


6

One should not confuse the "smearing" used to facilitate self-consistent field convergence, especially for density functional calculations in periodic systems, and the role of fractional natural orbital occupation numbers that arise naturally from the many-particle nature of the exact wave function. Natural orbitals are typically ordered in ...


5

CISD is not size-consistent for more than two electrons, and CCSD is size-consistent and size-extensive for any number of electrons. With CCSD, if you calculate the $\ce{Li2}$ energy for a bond length of 1000 Å, you should get the same thing as you would get by calculating the energy of a $\ce{Li}$ atom and multiplying it by two, and this would work for all &...


4

As far as I am aware, double hybrids were first proposed by Truhlar and coworkers in J. Phys. Chem. A 2004, 108, 21, 4786–4791. If you look at the paper, in addition to MP2 they also proposed a double hybrid based on QCISD, i.e. the quadratic configuration interaction method which can be viewed as an approximate coupled-cluster method. There's no reason a ...


3

Your CISD and CCSD energies are practically the same, since you have not defined a basis set. It appears Gaussian still defaults to STO-3G in this case, although it is a minimal basis set, which usually do not yield even qualitatively correct results even at the SCF level of theory. To be reliable, a post-HF calculation should employ basis sets of at least ...


2

The issue might be numerical as well. At theory level, CCSD could be rigorously prove to be size extensive. However, in practice, when one solves CC residue equation numerically, there is some convergence threshold that affect the accuracy of the result. Math is always simple and graceful but numerics can be complicated and nesty


1

An easier route to using symmetry-adapted CI with hand-derived matrix elements is to implement CI with determinant strings. That is, you construct bitstrings of which orbitals are occupied in the determinant, and you don't care about adapting your basis for $\hat{S}^2$. This is the way most codes work, since the resulting algorithm is easy to make very fast, ...


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