# Tag Info

Accepted

### Is density functional theory an ab initio method?

First note that the acronym DFA I used in my comment originates from Axel D. Becke paper on 50 year anniversary of DFT in chemistry: Let us introduce the acronym DFA at this point for “density-...
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• 42.3k

### Is density functional theory an ab initio method?

The convention used by many is that ab initio refers solely to wave-function based methods of various sorts and that first principles refers to either wave-function or DFT methods with little ...
• 26.9k
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### How does optimizing the molecular orbital coefficients in CASSCF improve its multi-reference capabilties?

I think you are maybe confusing how dynamical and static correlations are treated with different method. Also CASSCF by itself is not a multi-reference method. CI in general is able to describe both ...
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### Computing two-electron integrals with an STO-3G basis set

Actually there is a mistake in the analtical expression in Cook's Book. On his web page he has a pdf with the corrected verison http://spider.shef.ac.uk/ Maybe this solves your problem, but I would ...
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### Why are correlation consistent basis sets used with DFT calculations?

Because in fact it is appropriate. In most cases there is not a huge difference (quality/efficiency) among basis set families. For example Dunning (cc) basis sets work reasonable well for DFT, and ...
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### Initial guess for Unrestricted Hartree-Fock calculation

When $N_{\alpha} = N_{\beta}$, a restricted solution of Roothaan equations is a solution to the unrestricted Pople-Nesbet equations. This restricted solution always exists and necessarily results if ...
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### Gaussian09 vs home made Hartree-Fock implementation

Diagonalization of the core Hamiltonian provides usually not the best guess for the SCF procedure to say the least, and thus, by default Gaussian uses a more sophisticated guess obtained by ...
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### Proof of Brillouin's theorem using the diagonality of the Fock matrix

Your question actually contains two linked inquiries. The proof itself depends on the nature of the Fock-operator to which I'll have a few words afterwards. For the proof itself it is only important, ...
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### How does optimizing the molecular orbital coefficients in CASSCF improve its multi-reference capabilties?

The distinction between static and dynamic correlation is not well-defined.1 The distinction is only sensible with respect to a single-particle picture, i.e. viewing the many-electron wavefunctions as ...
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### What is the reason for including excited states in the wave function in post-Hartree Fock methods?

I think that here it is not accurate to say "starting from the Hartree-Fock picture", instead of it, I find better to say "starting from the Hartree-Fock method". When this method is carried out we ...
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### Ab initio effective potentials

First off, I would like to note that you wrongly interpreted the very first equation. Below I quote the relevant part of the paper: In the Hartree-Fock (HF) approximation for, say, the $\ce{Na}$ ...
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### How to choose the best method and basis set for a calculation in computational chemistry?

The experimental equivalent of your question would be: what kind and level of impurity am I going to accept in my experiment? In terms of basis sets: in principle you keep increasing the complexity ...
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### Boys function for Gaussian integrals in ab-initio calculations

I am not aware of any existing Fortran code for direct numerical quadrature of this problem, but it is worth pointing out that Mathematica can perform this integral symbolically: ...
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### Boys function for Gaussian integrals in ab-initio calculations

I know this is an old question, but I would like to give a small comparison regarding efficiency when evaluating the Boys function $F_n(x)$. Below are some implementations (in Julia) with simple ...
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### Computing two-electron integrals with an STO-3G basis set

My advice is to implement the Obara-Saika recurrence formulae that are outlined in "Molecular Electronic-Structure Theory" by Helgakar, et al. I would stick with Cartesian functions, since a) they are ...
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### Spectroscopic constants - explanation

You can find plots and further explanations online (as I easily did). This is a homework-type question, yet I do feel that the information request for diatomic molecular spectroscopic constants ...
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### What are typical runtimes for CASSCF calculations?

It seems increasing the active space helped make the CASSCF calculation converge. I've had success with increasing the number of active orbitals from 8 to 10. It only took an hour or so.
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### What's a good measure of the Eigen-ness of a wavefunction?

$\hat{F}\psi_i=\varepsilon_i \psi_i \implies \langle\phi_k|\hat{F}|\psi_i \rangle = \varepsilon_i\langle\phi_k|\psi_i \rangle$. So the Fock matrix $F_{ki}$ can be approximated by the product of ...
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### How closely should the energy components of ab initio SCF results at a stationary nuclear configuration preserve the virial theorem?

I hope I am not mistaken about this, but if I'm wrong then I'll learn so no big deal. I believe this is a result of the fact that the electrons do not experience a true $r^{-1}$ potential due to ...
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