Questions tagged [many-body-perturbation-theory]

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How do you calculate a full quartic force field for anharmonic frequencies?

I am an experimental chemist interested in anharmonicity—I am particularly interested in how to calculate anharmonic frequencies. The method I have the most experience with is (generalised) second ...
isolated matrix's user avatar
2 votes
1 answer
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Does the relaxed density (and therefore the relaxed orbitals) depend on the final energy in OO-DHDF+D calculations?

During an OO-RI-ωB97X-2-D2 calculation on ORCA, the non-dispersion-corrected OO-DHDF part of the calculation is performed, the D2 dispersion correction energy added to the resulting OO-DHDF energy, ...
Kanghun Kim's user avatar
6 votes
2 answers
360 views

Density Functional Perturbation Theory

What is the difference between density functional perturbation theory (DFPT) and many-body perturbation theory (MBPT)? Also, please help me understand why approximations like PBEsol are considered as ...
AbPhys's user avatar
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2 votes
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Is there an analogue of the K-S theorem for double hybrids

In my previous question, I was basically asking whether the results of the double hybrid using the exact XC functional are the same as those of just the exact XC functional. Even that sentence is hard ...
Kanghun Kim's user avatar
7 votes
1 answer
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Is an "exact" double hybrid density the same as the "exact" DFT density?

Double hybrid approximate functionals have "unoccupied" Kohn-Sham orbitals in their formulations due to their MP2 component; however, the "exact functional" depends only on the &...
Kanghun Kim's user avatar
2 votes
3 answers
132 views

Can I freeze f functions from a TZ basis set during MP2 (but not during H-F) to speed up calculations?

This is another question about my molecule; as I'm not doing this as an academical affiliate(I'm currently majoring in mathematics), I have only NWChem and my personal laptop on the mix, and thus need ...
Kanghun Kim's user avatar
5 votes
1 answer
102 views

How to plot downfolded bands from DFT bands?

Once you mapped the high symmetry points from primitive cell to the super cell, you need to plot the downfolded bands. As we know in DFT output of band structure calculation we get a set of E vs k ...
Alpha_Roy's user avatar
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8 votes
2 answers
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What does it mean when the first order correction energy is 0?

Suppose I have the following Hamiltonian to start $$ H_0 = \begin{pmatrix} 0 & 0 & 0 & 0\\ 0 & 0 & 2 & 0\\ 0 & 2 & 0 & 0\\ 0 & 0 & 0 & 0 \end{pmatrix} $$...
KAJ226's user avatar
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8 votes
1 answer
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What do size-extensivity and size-consistency mean?

I have heard both terms in various lectures and books on quantum chemistry, however, I have not found a proper explanation of them. As I understand now, size consistency of a method means for example, ...
S R Maiti's user avatar
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7 votes
1 answer
426 views

How does coupled cluster doubles (CCD) compare to MP2?

I have seen an increase in the use of the coupled-cluster doubles (CCD) method recently. CCD uses the exponential ansatz of the coupled cluster equations, but only includes amplitudes to double ...
jheindel's user avatar
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8 votes
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What are the types of PT applied to electronic structure methods?

The motivation for this question is similar in nature to the series of questions on different methods : What are the types of SCF? What are the types of MCSCF? What are the types of Quantum Monte ...
user avatar
6 votes
2 answers
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Geometry optimization with Moller-Plesset MP4

Can we optimize a molecular structure using Moller-Plesset MP4 method on GAUSSIAN software? If yes what keyword do we have to add?
sarra6's user avatar
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11 votes
2 answers
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DFT functionals from MP2 methods

I have heard people talk about DFT functionals built starting from MP2 corrections. Do you happen to have a reference I read about that from?
Yepman's user avatar
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15 votes
3 answers
763 views

How should I compare different basis sets for post-Hartree-Fock methods?

HF or DFT methods are variational so I can confidently say that any basis set that gives the lowest energy is the best for that system. However, I have learnt that post-HF methods such as MP2, MP3, ...
S R Maiti's user avatar
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14 votes
1 answer
205 views

Why are single excitations ignored in the MP2 component of double hybrid functional calculations?

In the original paper by Grimme introducing double hybrid functionals (also summarized in just 3 paragraphs here), it says: "[As] opposed to nonempirical versions of KS-PT2 [19], the single ...
Nike Dattani - No Free Time's user avatar
9 votes
1 answer
156 views

The correlation energy from frozen natural orbtials (FNOs) using CCSD and MPBT(2)?

In other works and from the paper: Scaling Up Electronic Structure Calculations on Quantum Computers: The Frozen Natural Orbital Based Method of Increments. I have seen that the corrected CCSD ...
Wychh's user avatar
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