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Questions tagged [basis-set]

A set of (basis) functions make up a basis set. These functions (generally) represent atomic orbitals whose linear combinations are used to make molecular orbitals (LCAO-MO) in computational chemistry.

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Hartree Fock for an atom (Beryllium)?

I'm working on solution of Hartree-Fock equation for Beryllium in configuration $1s^2 2s^2$, expanding the orbital with STO-3G basis $$\phi_k=\sum_{p=0}^3 C_{kp}e^{-\alpha_{kp}r^2}$$ I know the values ...
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What is the ANO-RCC basis set and how does it work?

On the basis set exchange, the ANO-RCC basis looks huge, even for hydrogen: ...
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Optimized General Contractions using the EMSL Basis Set Exchange

When using the EMSL BSE Database and selecting a basis set, one can choose the "Optimized General Contractions" or not. When doing so for, e.g., Nitrogen and the cc-pVDZ some basis functions are not ...
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How do the def2-SV, def2-SV(P), and def2-SVP basis sets differ?

I am trying to calculate the energy of complex molecules with DFT and have a question about the Ahlrichs (Karlsruhe) def2 basis sets. What is the difference between the def2-SV, def2-SV(P), and def2-...
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Reducible representations - choice of new basis

I was looking at this tutorial about reductions of representations. At first, the basis consists of four orbitals $(s_N, s_1, s_2, s_3)$ and the representations look like this and we want to reduce ...
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77 views

Total SCF Energy with Pseudopotentials

Is there a physical meaning to the SCF energy of a molecule obtained using pseudopotentials to represent the core electrons of heavy atoms (for example gold)? I've been running calculations in ...
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Reading CGTF basis from Basis Set Exchange site

I'm making a simple electronic structure code as an exercise, and I want to confirm that my understanding about basis set data from BSE is correct. The CGTF of H in aug-cc-pVTZ basis is given as: H ...
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218 views

Minimal basis for Li

Currently, I'm trying to get into the field of quantum chemistry and was reading about minimal basis sets. As I understood this is the minimal number of orbital/spatial wavefunctions needed for a ...
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220 views

Pople Basis Set Abnormality: 3rd row 6-311G

I noticed when looking through the Basis set exchange website that the 6-311 Pople basis sets don't at all match their formulation once you go past the 2nd row of the periodic table. For example, ...
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183 views

How to form 6-31G basis set; specific instructions

I'm new to basis sets. I read that for 3-21G, given the following basis set for carbon (from Basis Set Exchange): ...
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1answer
80 views

Why is a set of p-functions added to the minimum basis sets of Li and Be?

The minimum basis set consists of the smallest number of functions necessary to contain the electrons of the neutral system. In the book Introduction to Computational Chemistry, Second Edition, by ...
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Use of basis set in DFT (Density Functional Theory)

Basis sets are used to guess the electronic wave functions for Hartree Fock or similar methods, which are quite legitimate since these methods deal with the wave function of each and every electron. ...
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How to determine which p-type GTO to choose in HF method?

I am trying to understand the working principle of basis sets in quantum chemistry. As far as I understand in the Hartree-Fock method, the basis set is used to calculate the matrix representation of ...
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Visualizing s and p orbitals from contracted basis set

I was thinking about basis sets used in Quantum Chemistry programs and thought of why not try to visualize them. I started with GTO from EMSL website for $\ce{H2}$, specifically cc-pVDZ basis which ...
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How to compute 2-electron integral for Hartree-Fock code?

I'm working on writing my own code for $\ce{H2}$ in an STO-3G basis set using Hartree-Fock (HF), and I am currently stuck on how to construct the two-electron integral matrix. I know how to evaluate ...
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310 views

Largest system studied by full CI

What is the largest system (organic molecule) which total energy was calculated by full configuration interaction? Lets say with DZV or similar basis set. How far can we get with CI in larger basis ...
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759 views

The eigenvector matrix C

I was wondering what the difference between $c_n$ and $c_{jn}$ is? In the context of molecular orbital construction, for example, if $c_{jn}$ is the coefficient of the basis functions $\phi_j$ (...
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Are the coefficients and exponents of contracted gaussian same for cartesian and pure form?

The cartesian form of contracted gaussian is something like $$\phi^{CGF}=\sum_{n} s_nx^iy^jz^k \exp(-\alpha_nr^2)$$ and these is also a pure or spherical harmonic version (which I don't know the ...
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How does one compute the number of unique 2-electron integrals for a given basis set?

I have run into trouble with an exercise in Szabo and Ostlund's Modern Quantum Chemistry. Exercise 3.14: Assume the basis functions are real and use the symmetry of the two-electron integrals [$(\...
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570 views

“NBO diagrams” versus MO diagrams

Molecular orbital diagrams are well known by chemists. According to Wikipedia (emphasis mine), A molecular orbital diagram, or MO diagram, is a qualitative descriptive tool explaining chemical ...
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268 views

problem with loops over basis sets in psi4 using python

I've recently started using the psi4 quantum chemistry software. I wanted to compare a couple of different basis sets (mainly pople style) on an amino acid. So I've written a python code inside the ...
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497 views

Matching a Slater-type wavefunction with a minimal (STO-nG) Gaussian basis set

I have on hand a high-precision wavefunction expressed in Slater orbitals. I need to express it as accurately as possible in a minimal Gaussian basis set. For background: I am currently working with ...
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Why are basis sets needed?

I am not sure whether this question is even reasonable, but here it goes. We are taught about the different types of basis sets (extended, minimal, double-zeta, plane wave), but I do not think it is ...
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Accuracy of FP-LAPW vs Plane Wave Pseudopotential: examples

I am trying to learn about FP-LAPW software. I would like to know how well FP-LAPW outperforms Pseudopotential PW calculations. What are some properties/calculations/quantities in which FP-LAPW gives ...
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Why are correlation consistent basis sets used with DFT calculations?

Reading some papers, there is a bit of a curious observation that does not exactly make a lot of sense to me. In Quantum Chemistry, different methods exist to carry out geometry optimisation and ...
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Why does bond length decrease with increased basis set quality for SCF methods?

From Reviews in Computational Chemistry Vol. 5, for $\ce{H2O}$ using HF, MP2, CCSD, and CCSD(T), all lengths in angstroms: $$ \small \begin{array}{rcccc} \text{Basis} & \text{HF} & \text{MP2}...
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Equivalent of Szabo and Ostlund book for DFT

Szabo and Ostlund book Modern quantum chemistry [1] is extremely useful to understand Hartree-Fock and post-Hartree-Fock methods. Not only it explains the theory behind such methods, but it is also ...
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How to choose the best method and basis set for a calculation in computational chemistry?

In Quantum chemical calculations, it is not always the case that the more complex (correlated) the method or the basis set the more accurate the results. For example, it's sometimes found that MP2 ...
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Boys function for Gaussian integrals in ab-initio calculations

A few days ago I mentioned a problem with an Hartree-Fock program I am writing (HF using cartesian Pople's STO-3G basis set). I can reproduce overlap and kinetic integrals of some references for ($\ce{...
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3k views

How many basis functions used in STO-3G and 6-31+G** for the water molecule?

I am a little confused about how to decide how many basis functions that are used in a particular basis set for a given molecule. STO-3G is a so-called "minimal basis set", meaning that only one ...
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1answer
521 views

Can PBE (and LDA) actually be a better choice sometimes?

Part of my work as an inorganic chemist is to investigate the magnetic coupling between metallic centers in coordination compounds. After some time, I've noticed that the classic PBE functional is the ...
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3answers
1k views

Computing two-electron integrals with an STO-3G basis set

I am trying to implement a restricted Hartree-Fock calculation using an STO-3G basis set, for fun. I managed to perform this calculation where only $\mathrm{1s}$ orbitals are present ($\ce{H2}$ and $\...
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Why large basis sets give better approximations to the exact solution of the Schrödinger equation?

The variational principle states that the energy of any approximate wave function will always be equal to or greater than the energy of the exact solution. Therefore the energy is minimized when ...
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Which Basis Set is suitable For Mercury-complex in DFT calculations?

Which Basis Set is suitable For Mercury-complex in DFT calculations? Please provide the answers with any journal reference.
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Coefficients and Parameters for contracted Gaussian basis sets

I'm trying to write a program to calculate fixed-point Hartree-Fock level energies of molecules (for my amusement) and everything makes sense but this. I've been agonizing over this for almost 3 hours ...
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An alternative basis set for analytical computation of two electron integrals

It is well known that the usage of the Gaussian basis set, in contrast to Slater basis set, leads relative simple semi-analytical expressions for the two electron repulsion integral $(ab|cd)$. Could ...
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Which computational method/basis set should I use for small molecular ions?

I apologize for a perhaps newbie question. I would like to compute the potential energy surface for several small molecular ions, like $\ce{H2+}$ and $\ce{H3+}$, using quantum chemistry software. ...
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857 views

Naming of quantum chemistry basis sets

For some DFT calculations I performed with localized (Gaussian) basis sets, I wonder how the basis sets I used are properly named. The names I could gather are: H, C and O: 6-311G(d,p) Al: 8-511G* ...
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Are Pulay forces expensive to compute?

In a thesis I am reading, it is said that one of the reasons for using plane-wave basis sets for first-principles molecular dynamics (aka ab initio MD) is that the Pulay forces[1,2] that arise from an ...