39 votes
Accepted

Was Walter Kohn wrong about this?

Kohn is easily one of my favorite humans of all time, and he was a role model to whom I looked up in great admiration for most of my academic life; in fact before this site was created, I proposed ...
Nike Dattani - No Free Time's user avatar
36 votes
Accepted

Approximate equivalence table between Pople's basis sets and Jensen's DFT optimized polarization consistent basis sets (pcseg-n)

Disclaimer and warning: long and likely biased answer. Background: The Pople style basis sets were defined almost 50 years ago. The 6-31G was designed for HF calculations, the 6-311G for MP2 ...
Frank Jensen's user avatar
  • 1,776
32 votes
Accepted

Where is the extended Hückel method (EHM) still used today?

In an era of ab initio methods and many-body methods like the $GW$, there is not too much room for methods like the extended Hückel model to be the main method in any particular field of materials ...
epalos's user avatar
  • 4,789
25 votes
Accepted

What are the "smart algorithms" applied to solve the "curse of dimensionality"?

This is a very broad question, so I am going to give a very brief overview of typical exponentially-scaling problems. I am not an expert in most of these areas, so any suggestions or improvements will ...
Godzilla's user avatar
  • 1,918
25 votes

What is a good replacement for Gaussian?

ORCA Official website: https://orcaforum.kofo.mpg.de/app.php/portal What is ORCA? ORCA is a general-purpose quantum chemistry program package that features virtually all modern electronic structure ...
Jack's user avatar
  • 15.1k
24 votes

What is a good replacement for Gaussian?

OpenMOLCAS MOLCAS is now in it's 5th decade! The first version was written in the 1980s by the group of Bjorn Roos, one of history's giants in quantum chemistry. It can do many of the things most ...
Nike Dattani - No Free Time's user avatar
22 votes
Accepted

Materials Modeling with Raspberry Pi?

Raspberry Pi clusters are okay for studying networked systems and job schedulers, but bad for any real calculations. There are several problems: there's very little memory per CPU, the interconnect is ...
Susi Lehtola's user avatar
  • 19.1k
22 votes
Accepted

When and Why does Density Functional Theory (DFT) fail?

First, a word of caution: it is hard to generalize since there are so many different approximations to the exact exchange-correlation functional. Nonetheless, in my opinion: The biggest weakness of ...
Roman Korol's user avatar
  • 1,073
21 votes

When and Why does Density Functional Theory (DFT) fail?

It is very important to differentiate between Density Functional Theory (DFT) and Density Functional Approximation (DFA). DFT is an exact theory and if we know the exact formulation for exchange-...
Thomas's user avatar
  • 9,072
21 votes
Accepted

Is there a set of updated, comprehensive benchmarks for speed comparison between different quantum chemistry packages?

The problem is that this is a highly volatile question. In order to meaningfully benchmark programs, you have to use the exact same compiler flags (may require heavy hacking) and use the same ...
Susi Lehtola's user avatar
  • 19.1k
20 votes
Accepted

Total spin and/or multiplicity for transition metal ions?

Even for the simplest transition metal diatomic molecules, the most reliable way to know the ground-state spin configuration is often by doing experiments. I will give an example where it's easy to ...
Nike Dattani - No Free Time's user avatar
20 votes

Correct jobs sequence for transition-state (TS) search

A common and not quite unreasonable approach is to do molecular structure optimisations at a low level, establish the path(s) at this level, and obtain better energies at a higher level. It really ...
Martin - マーチン's user avatar
20 votes

Is it better for me to study chemistry or physics?

I just joined this community to answer you after I've randomly seen your post, because I think there are a couple of important variables you may want to consider: your feelings and your intuition. You ...
Alf Pascu's user avatar
  • 301
19 votes

How can quantum computing accelerate materials modeling?

Quantum computers provide the possibility of simulating systems that are so mathematically complex that classical computers cannot practically be used. The difficulty comes from the fact that the ...
Peter Morgan's user avatar
  • 1,483
19 votes
Accepted

When do we abandon ab initio methods?

There's three scenarios that come to my mind, for when ab initio methods get abandoned: The cost becomes prohibitive (e.g. too many electrons) The insight is lost It is simply not required for what ...
Nike Dattani - No Free Time's user avatar
18 votes

Where is the extended Hückel method (EHM) still used today?

One area where the extended-Huckel method continues to see use is to form the initial guess for an SCF calculation or even just a more accurate semi-empirical method. While most electronic structures ...
Tyberius's user avatar
  • 15.5k
17 votes
Accepted

Since MKL is not optimized for AMD hardware, should I use a math library specific to AMD, or would an open-source one be just as good?

OpenBLAS is a free, open-source BLAS library that has fast support for even recent processors. (It is based on the earlier, famous GotoBLAS library which became obsolete years ago.) OpenBLAS is also ...
Susi Lehtola's user avatar
  • 19.1k
17 votes

What is a good replacement for Gaussian?

Psi4 Psi4 is a fully open-source quantum chemistry package that is gaining a lot of interest in my community (computational) combustion chemistry. Here is the manual for the latest release: it has ...
DoctorSoup's user avatar
16 votes

What is a good replacement for Gaussian?

GAMESS For many researchers, GAMESS is the "natural" substitute for GAUSSIAN. Developed and maintained by the group of prof. Gordon (also a banned GAUSSIAN user, see image below). Official ...
Camps's user avatar
  • 23.3k
16 votes

I am considering to purchase a GPU. What calculations would need FP32, and what would need FP64?

Numerical Feynman integrals I wrote a software to "exact quantum dynamics" calculations, meaning that given a density matrix of a system under the influence of noise at one point in time, ...
Nike Dattani - No Free Time's user avatar
15 votes

How can quantum computing accelerate materials modeling?

Quantum computing has been extensively discussed as long as the first quantum computers have become a reality. As already pointed out, we are still at very early stages of such development. What we ...
Anibal Bezerra's user avatar
15 votes
Accepted

Does the Schrödinger equation have unique solutions?

The solutions to the Schrödinger equation are not unique in general, and uniqueness depends on several things such as the form of the potential and boundary conditions. Many papers have discussed ...
Nike Dattani - No Free Time's user avatar
15 votes
Accepted

Gaussian-type orbitals vs Slater-type orbitals in terms of computational efficiency

Your supervisor is correct that in almost all practical cases in quantum chemistry, the cost savings of using Gaussian-type orbitals far outweigh the disadvantages of needing more orbitals. First of ...
Nike Dattani - No Free Time's user avatar
14 votes

Where is the extended Hückel method (EHM) still used today?

I know of at least one place, where it is relatively common to use the extended Hückel theory in practise: in generating initial guess orbitals for further electronic structure calculations. The most ...
Martin - マーチン's user avatar
14 votes
Accepted

Basis sets optimized for CCSDT(Q) calculations

To the best of my knowledge, there have not been basis sets optimized for post-CCSD(T) calculations. Note that the cc-pVXZ have been optimized using CISD. One would expect these to be also close to ...
Frank Jensen's user avatar
  • 1,776
13 votes

How can quantum computing accelerate materials modeling?

Quantum computers potentially could provide a far more efficient way to solve the electronic structure problem: finding the ground state energy (or other properties) of electrons in a compound. The ...
Max Radin's user avatar
  • 231
13 votes

When and Why does Density Functional Theory (DFT) fail?

Known failures of density functional approximations (DFAs) include anions, charge transfer systems and point defects (e.g. vacancy states). These are mainly due to self-interaction error, which can be ...
Susi Lehtola's user avatar
  • 19.1k
13 votes
Accepted

How is MCTDH derived?

I will outline the way it was derived in the original 1990 paper. We start with an ansatz for the time-dependent wavefunction: \begin{equation} \tag{1} \psi(x_1,\ldots,x_n;t) = \sum_{j_1=1}^{m_1}\...
Nike Dattani - No Free Time's user avatar
13 votes

Is it better for me to study chemistry or physics?

As someone who did his undergrad in Canada, PhD in UK, and acquired post-PhD research experience in Japan, Singapore, USA, Germany, and again in Canada: I can say that the dilemma you're facing is a ...
Nike Dattani - No Free Time's user avatar
13 votes
Accepted

Convert reduced density matrix from M.O. to A.O. basis

Only the first line is correct, as long as the AO basis is not orthogonal. The point is: the AO $\leftrightarrow$ MO transformations of density matrices and Fock matrices require different formulas. ...
wzkchem5's user avatar
  • 9,620

Only top scored, non community-wiki answers of a minimum length are eligible