Questions tagged [computational-chemistry]

For questions about computational chemistry.

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17
votes
6answers
355 views

What are the applications of chemical graph theory?

Graph theory was originally introduced in computer science to study data structure. Chemists also introduced graph theory to study the relation between structure and properties for molecular compounds....
9
votes
1answer
400 views

High precision helium energy

In the paper "High Precision Theory of Atomic Helium", Drake lists the then best-known ground state energy of the Schrödinger equation for helium to 22 digits as: $$\lambda_0 \approx -2....
25
votes
3answers
1k views

What does B3LYP do well? What does it do badly?

I have seen a general criticism of B3LYP in computational chemistry (especially with the 6-31G* basis set). I would like to know some of the more/less obvious problems that occur when using it and ...
13
votes
2answers
329 views

How to get potential energy surface from GAMESS?

I'm trying to learn GAMESS and performing a simple PES scan of oxygen molecule. After the completion of the calculations I get a .log file with lots of sections like that: ...
8
votes
1answer
1k views

What is the physical meaning of occupations='smearing' in Quantum ESPRESSO?

I want to understand, what is the intuition behind using the occupations='smearing' or 'tetrahedra' in Quantum ESPRESSO input file? I mean how we can interpret what is going to happen inside of ...
12
votes
1answer
94 views

How can one computationally determine the flammability of a molecular system?

I can imagine one aspect of flammability (the ability of a substance to ignite on fire) to include calculating the kinetics and thermodynamics of the reaction of the system with oxygen. This is not ...
13
votes
4answers
226 views

What is the role of a chemist in computational chemistry?

As someone that wants to learn about computational chemistry, mainly for its use in energy storage materials (and therefore DFT as the most practical method of it), I am wondering what is essential ...
8
votes
1answer
77 views

Excited states using ORMAS flag in GAMESS

I am trying to calculate the excited states using single reference CISD on $\ce{CH3OH},$ using ORMAS' CI-X option in GAMESS. As a test example I am using the $\ce{CH3OH}$ that has 2 core, 7 occ. ...
13
votes
2answers
166 views

Analytic Hessians for meta-GGA functionals

In many of the free/open-source QM programs like GAMESS, ORCA, NWChem, the calculation of analytic hessians is not possible when a meta-GGA density functional is used, like M06-2X. My first question ...
11
votes
2answers
260 views

One-center two-electron integrals between 1s STO

As per @user1271772 's suggestion I am asking this question here again. I am trying to understand SCF cycle by trying to code up solved example from Quantum Chemistry by Levine (page 443, 5th edition)...
10
votes
1answer
136 views

What are the trade-offs setting DIRSCF to either .TRUE. or .FALSE. (the default) in GAMESS (US)?

The first software I used to run calculations was Gaussian09. When submitting a input, I used to see a pattern of resource usage where the allocated CPU quickly maxed out, remaining high most of the ...
10
votes
3answers
549 views

Which DFT functional should I use to simulate spectra of pesticides?

I am mainly searching for functionals to calculate spectra of pesticides. I found the benchmark test from Goerigk and Grimme$^1$, but don't understand which of the functionals suits my purpose the ...
8
votes
1answer
218 views

Which basis set should be used for a system that contains both anions and cations?

I am trying to model a system that has both anions and cations interacting with each other. (In this case it is calcium ion interacting with two small carboxylic acid molecules). Now, looking through ...