# Tag Info

33

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 calculations. For computational efficiency reasons, the s- and p-exponents were constrained to be identical. Polarization functions were defined for 1d, 2d, 3d and 1f. ...

32

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 that we name it after him. However I completely disagree with the sentence that you have quoted. Keep in mind that even though the Nobel Lecture was in 1999, Kohn ...

30

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 modeling. However, the method is still very much appreciated by the solid-state community particularly for it's accessibility. It is especially popular with those ...

24

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 be welcome. Solving the Schrödinger equation In order to solve the Schrödinger equation numerically, you need to diagonalise a rank $3N$ tensor -- as you can ...

22

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 methods (density functional theory, many-body perturbation, and coupled-cluster theories, and multireference and semiempirical methods). It is a flexible, ...

22

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 mainstream quantum chemistry packages can do: integrals, Hartree-Fock, DFT, MP2, coupled cluster, geometry optimization, etc., and while it can do "single-...

19

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 slow, having local disk is hard... but worst of all, the bang per buck is very low, see e.g. a Phoronix benchmark. So, in summary: Intel/AMD is still cheaper ...

19

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 algorithms and parameters (accuracy, cutoffs, quadrature grids, etc). But, if a program supports many kinds of algorithms, then each of them would have to be ...

19

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 can rationally consider many variables, but you are very young and there may be topics that you like now that you will consider boring in the future, and the ...

18

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 packages use the Superposition of Atomic Densities (SAD) guess as the default, the option is available in almost all of them and Psi4 uses it as the default for ...

18

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 all existing (and arguably all plausible) implementations of DFT is their limited predictive power. In practice, this means that you need to know a lot about ...

17

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-correlation functional, we should get the exact solution. However we don't have the exact formulation and hence we choose different approximations for it, ...

17

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 multi-platform: in addition to x86 and x86_64 it also supports other architectures like ARM and PowerPC. OpenBLAS also has runtime CPU detection; if you compile ...

16

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 electrical properties of materials, and other chemical systems, are governed by the laws of many body quantum mechanics, which contain equations that are extremely ...

16

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 we want to do Prohibitive cost: If solving the Schrödinger equation (for example) is no longer possible, we may very well still wish we could solve the ...

16

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 most, if not all, of the theoretical methods (HF, DFT, CC,...) and also lots of interfaces and integrates into C++ and Python (sadly no Fortran). The bonus on it ...

15

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 correctly determine the ground state spin configuration, and then an example where it has remained impossible as of the year 2020. Cr$_2$: Here we can accurately ...

15

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 uniqueness of solutions to the Schrödinger equation for specific classes of potentials and boundary conditions, but in general it is possible to come up with cases ...

15

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 website: https://www.msg.chem.iastate.edu/gamess/index.html What is GAMESS: GAMESS is a program for ab initio molecular quantum chemistry. Briefly, GAMESS can ...

14

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 popular example I can think of is Turbomole, see its manual (pdf, chapter 4.3, p. 75). They claim that the starting vectors are better than a core Hamiltonian ...

13

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 need to keep in mind is that a quantum computer needs a quantum computing algorithm. The latter can have few to no correlation with classical algorithms, such as ...

13

I will outline the way it was derived in the original 1990 paper. We start with an ansatz for the time-dependent wavefunction: $$\tag{1} \psi(x_1,\ldots,x_n;t) = \sum_{j_1=1}^{m_1}\cdots \sum_{j_n=1}^{m_n}a_{j_1\cdots j_n}\phi_{j_1}^{(1)}(x_1,t)\cdots \phi_{j_n}^{(n)}(x_n,t),$$ with single-particle functions (SPFs) satisfying (...

13

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 major disadvantage of the undergrad UK system. The UK system has many advantages too! But the flexibility is better in other countries such as Canada and US. ...

12

The curse of dimensionality is indeed a huge problem in quantum chemistry, since the possible ways N electrons can occupy K orbitals is a binning problem whose computational cost grows factorially (almost as fast as x^x!) with the size of the system. Moreover, for accurate results you need K>>N in order to account for the so-called dynamical correlation, ...

12

I think the way this question is asked is a little too simplistic. In order to execute a computational project, there is always more than one calculation required. Even if you are happy with the lowest level of theory (say, B3LYP/6-31G*), it does not mean that any package that lists B3LYP in the list of the available features would be useful. Possible ...

12

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 mitigated to some level with hybrid functionals and range-separated hybrids. DFAs are also generally unreliable for systems with strong correlation, like many ...

12

The 2006 variational calculation by Schwartz is lower (more accurate) than Nakashima & Nakatsuji's 2007 energy: 2006 Schwartz: -2.903724377034119598311159245194404446696925309838 2007 Nakashima & Nakatsuji: -2.90372437703411959831115924519440444669690537 The lowest variational upper bounds ground state energies for the first 6 elements, ...

12

As a Bachelor in Physics that do research in Material Science and in Drug Development, I have to recommend you to go for the Physics degree. Also, you have to be aware about the type/specialization of the Physics course. For example, my undergrad Physics course was focused in Solid State, so, we had Quantum Mechanics, Solid State Physics and Crystallographic,...

12

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 all, there is a bit of a misconception that Slater-type orbitals are far more accurate. The motivation for using Slater-type orbitals is due to their resemblance ...

12

These are called double hybrid functionals. You add a fraction of MP2 correlation on top of the DFT energy. A good reference is e.g. the review by Goerigk and Grimme from 2014 published in WIREs 4, 576 (2014). Note that this is already 7 years old; many double hybrids have appeared since then. However, it should give you a good overview.

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