Questions tagged [quantum-chemistry]

For questions about quantum chemistry.

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32
votes
1answer
2k views

Was Walter Kohn wrong about this?

In Kohn's Nobel Lecture, he claimed that: "In general the many-electron wave function $\Psi(\mathbf{r}_1,\ldots,\mathbf{r}_N)$ for a system of $N$ electrons is not a legitimate scientific concept,...
30
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5answers
2k views

What is a good replacement for Gaussian?

When I started studying computational chemistry (circa 2007), my supervisor used to tell me about the controversy surrounding Gaussian, Inc. regarding the banning of researchers involved in the ...
27
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1answer
544 views

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

At first I posted this question in chemistry.stackexchange.com, before materials.stackexchange.com was launched in beta. As it went unaswered, I'm giving a try here. I have some calculations results ...
26
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3answers
291 views

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

The extended Hückel method (EHM) proved to be very useful through time, but there are better and affordable models today. One interesting thing about the model is the independence of the Hamiltonian ...
24
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2answers
2k views

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

The "curse of dimensionality" is an ubiquitous issue arising in both electronic structure and quantum molecular dynamics, which refers to the exponential scaling of computational cost with ...
23
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5answers
506 views

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

Every method has its strengths and weaknesses. For instance, a strength of DFT is that is has HF like speeds, but can also account for electron-correlation and this is a pretty big feature since ...
20
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3answers
166 views

How can quantum computing accelerate materials modeling?

Materials modeling is very computationally intensive and first-principles simulation of a real system of reasonable size typically involves the use of classical supercomputers. How can quantum ...
19
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3answers
1k views

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?

The codes for quantum mechanical calculations make heavy use of linear algebra, and it seems most of them delegate this task to time-tested and highly optimized libraries, instead of trying to deploy ...
19
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2answers
327 views

What significant matter modelling methods are implemented in commercial software, for which there is no freeware alternative?

There is an ever-growing list of freeware and open-source software for solid-state physics and quantum chemistry. But many commercial programs still thrive, even in 2020, and their cost can be in ...
16
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12answers
6k views

Is it better for me to study chemistry or physics?

I might decide to pursue material science. Aptitude and background I am from the UK, and for A-levels, I am studying Maths, Further Maths, Physics and Chemistry, and I am predicted decent grades in ...
16
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4answers
398 views

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

I've found a page dedicated to quantum chemistry packages benchmarks, on GitHub, qmspeedtest. But most results there are several years old, and so probably outdated. Is there some place where we can ...
16
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1answer
1k views

Does the Schrödinger equation have unique solutions?

I am learning DFT and the Hohenberg Kohn Theorem of Existence. It says that there is a one-to-one correspondence between the external potential and the density. However the proofs that I have seen ...
15
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3answers
2k views

What does “strongly correlated” mean?

For quantum many body problem, there is a common terminology “strong correlated systems” that appears in different context. However, it seems that the definition of it is ambiguous and sometimes ...
15
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2answers
370 views

Materials Modeling with Raspberry Pi?

Does anyone have any knowledge or direction for creating a homemade Beowolf cluster for materials modeling?I would like to be able to run my own "hobby" simulations at home, so a low energy, ...
15
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3answers
133 views

Candidate structures for global minimum determination

What are the best packages that can generate candidate structures for quantum chemical optimization (DFT or CCSD) to find the global minimum? I am working on the gas-phase small inorganic clusters (...
15
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2answers
98 views

What is the difference between orbital localisation and NBO?

I am new to computational chemistry, I have used mainly GAMESS and Orca. In GAMESS, there are orbital localisation schemes like Pipek-Mezey, Ruedenberg-Edminston, etc. which generate orbitals that ...
14
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1answer
99 views

Why are Norm-conserving Pseudopotentials considered so important?

So I am aware that a norm-conserving pseudopotential enforces the conditions that: Inside the cut-off radius, the norm of each pseudo-wavefunction is identical to its corresponding all-electron ...
14
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1answer
103 views

Why are single excitations ignored in the MP2 component of double hybrid functional calculations?

In the original paper by Grimme introducing double hybrid functionals (also summarized in just 3 paragraphs here), it says: "[As] opposed to nonempirical versions of KS-PT2 [19], the single ...
13
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1answer
185 views

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

When creating molecular orbitals via the linear combination of atomic orbitals method I am told that Slater orbitals provide a more accurate representation whilst Gaussian orbitals are computationally ...
13
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1answer
70 views

In which situations/problems, does one need to employ the nonorthogonal configuration interaction method?

Nonorthogonal configuration interaction is known to be a computationally inefficient approach. Recently, there has been a number of developments that have been done to improve the efficiency of ...
13
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1answer
90 views

What is a good practical way to determine whether or not I need to use a multiconfigurational method?

I’m planning to run a calculation of single electron transfer where my system goes from singlet to triplet biradical (presumably). So I’m wondering how to decide if I need to use milticonfigurational ...
13
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2answers
167 views

A CCSD(T) geometry optimization in Gaussian09 is deviating too far from the minimum. How do I deal with it?

I am doing a CCSD(T) geometry optimization on a series of molecules. One of the geometry optimizations took about 2 weeks even though an analogous molecule with the same number of basis functions took ...
12
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2answers
619 views

Total spin and/or multiplicity for transition metal ions?

I am trying to input various charge states of various transition metal ion species into DFT calculations. The program requires an input of multiplicity = 2*(total spin) + 1. I have an undergraduate-...
12
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1answer
521 views

When do we abandon ab initio methods?

This question is related to (and was originally asked in) another post about "quantum protectorates" I made here. Ab initio methods are nice because they directly solve a sort of "...
12
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2answers
66 views

What are some of the advantages and disadvantages when using shape consistent pseudopotentials as compared to energy consistent pseduopotentials

From the literature as well as the answer to the question What is the meaning of energy-consistent and shape-consistent in the context of pseudopotentials? I can see that the key difference between ...
12
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1answer
399 views

Are there examples of ab initio predictions on small molecules without the “major approximations”?

In most textbooks the precise Schrödinger euqation of a molecule is given (and the "beautiful thing" stops here), then born oppenheimer approximation is made, then layers of other ...
12
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1answer
184 views

How to explain Density Functional Theory results to an experimentalist?

When we present our Density Functional Theory simulation results e.g. lattice parameters, stacking fault energies, band gaps, etc. to people who are experimentalists then the very first question ...
11
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3answers
1k views

What does “level of theory” mean?

I was reading this paper on the QM9 synthetic molecular database and their computed properties. It is stated that All properties were calculated at the B3LYP/6-31G(2df,p) level of quantum chemistry. ...
10
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2answers
89 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 ...
10
votes
3answers
148 views

CCSD and CISD size-consistency problem

I have performed a Gaussian16 calculation to explore the size-consistency problem for the CISD method. The system I am studying is the water dimer and the results are not what I expected at first. I ...
10
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1answer
265 views

Does One-to-One Correspondence of Hohenberg Kohn Theorem Mean Bijective or Injective and How to Prove it?

I have asked a similar question but after thinking about it I have a more specific question. According to Ullrich, Carsten A.. Time-Dependent Density-Functional Theory : Concepts and Applications, the ...
10
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1answer
67 views

How to understand the formation of covalent bond from the viewpoint of wavefunction interaction?

The formation of the covalent bond is usually understood from the electron sharing, like explained by the following figure: However, this simple physical picture seemingly hinders many things. In ...
10
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1answer
85 views

Regarding oscillatory strength theoretical units to experimental ones

The output of Gaussian rotatory and oscillatory strength intensities, plus a gaussian/lorentzian fit, translates to a theoretical CD/UV-vis spectra. In order to try and compare with experimental ...
10
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1answer
116 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
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1answer
77 views

How to determine occupied and closed orbitals for a Molpro CASSCF calculation?

I am currently working with Molpro, to which I am completely new, trying to run some calculations on $\ce{PdO}$. From literature I have gathered that the ground state is given by $^3\Sigma^-$ state, ...
10
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0answers
83 views

How does one project out the translations and rotations from a hessian?

I've recently written a simple code to numerically compute the hessian of some function (at a point). Most electronic structure packages will compute the hessian and then project out the translations ...
9
votes
1answer
383 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....
9
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2answers
239 views

Normalization constant and Roothaan Equations

Roothaan Hall Equations: The Hartree-Fock equations are a set of modified Schrodinger equations: $f_{i}\psi_{m}=\epsilon_{m}\psi_{m}$ where: The Fock operator ($f_{i}$) is given by (restricted case): ...
9
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1answer
209 views

How can I implement the Cartesian harmonics?

I'm studying the quantum chemistry calculation and now trying to implement the basic methods. For example, given a water molecule, $M = {(\ce{O}, R_\ce{O}), (\ce{H}, R_\ce{H}), (\ce{H}, R_\ce{H})}$, ...
9
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1answer
73 views

What are some good ways to model radicals and similar intermediates of reactions

Calculations in the ground electronic state are probably the simplest cases where defaults in terms of charge (typically zero) and spin (singlet) are chosen by the calculation software. However, when ...
9
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1answer
105 views

Energy decomposition analysis (EDA) at transition state

I am trying to understand the interaction between the nucleophile and the substrate for some SN2 reactions (e.g. $\ce{I- +CH3Cl->CH3I + Cl-}$) at the transition states. Now, it seems that EDA would ...
9
votes
1answer
42 views

How can I determine the maximum value of a spin orbital and its derivative in any direction?

I'd like to be able to recover the maximum value that any of the basis functions can take, as well as their maximum derivative. How can I do this? For context, I need it because I want to calculate ...
9
votes
1answer
58 views

Is there an anisotropy factor (g factor) for TDDFT absorption and circular dichroism calculations?

Experimentally, anisotropy factor is calculated by dividing the CD (circular dichroism) spectra by the absorbance spectra and multiplying by a factor of 32980 (in order to get a dimensionless quantity)...
8
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1answer
108 views

How to get the .47 file from ORCA for natural bonding orbital analysis

I am trying to perform NBO analysis with relaxed density matrix with the open-source program JANPA. I am following the guide here. The first step is to get the NBO input .47 file from Orca v4.2.1. The ...
8
votes
1answer
883 views

How to fix Error Link9999 in Gaussian

I have a problem with geometry optimization in Gaussian software with M062X method. The structure was smoothly optimized using M062X/Gen. Nevertheless, I need very accurate results so afterwards the ...
8
votes
1answer
103 views

The correlation energy from frozen natural orbtials (FNOs) using CCSD and MPBT(2)?

In other works and from the paper: Scaling Up Electronic Structure Calculations on Quantum Computers: The Frozen Natural Orbital Based Method of Increments. I have seen that the corrected CCSD ...
8
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1answer
59 views

Error running psi4 on charged molecules

I am running a simple energy calculation on a conformer of 3-Sulfopropyl methacrylate, which is given my C=C(C)C(=O)OCCCS(=O)(=O)[O-]. This is my workflow: ...
8
votes
1answer
122 views

Is this G16 molecular hyperpolarizability error reproducible in other software?

I was trying to calculate the hyperpolarizability $\beta$ of a cluster of two water molecules in Gaussian16 using the route card: ...
8
votes
0answers
138 views

How to interpret second order perturbation theory analysis from NBO calculations? [closed]

The question is quite simple — how to interpret NBO results of second order perturbation theory analysis? As I have read several papers the most important interactions are $\pi\rightarrow\pi^{*}$, $LP\...
7
votes
1answer
81 views

Avogadro for Linux — Input Generators and Submitting Job to QM software?

Regarding input generators for a Linux (Linux Mint 20.1 Xfce) version of Avogadro2, I have managed to get this working using the Flatpak version and copying the input generator python scripts from ...