Questions tagged [quantum-chemistry]

For questions about quantum chemistry.

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7
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0answers
63 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 ...
9
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1answer
335 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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1answer
37 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
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2answers
35 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
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2answers
208 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): ...
8
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1answer
93 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 ...
5
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1answer
84 views

What are the other matrix elements in singlet symmetry-adapted CISD?

Szabo/Ostlund list the CI matrix elements between singlet symmetry-adapted configurations (SAC) in Table 4.1 of their book: $$ \langle ^1\Psi^r_ a \lvert \mathcal{H} - E_ 0 \rvert ^1\Psi^r_ a \rangle, ...
14
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3answers
103 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 (...
8
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1answer
99 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: ...
9
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1answer
64 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 ...
6
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1answer
64 views

I get an error 'Conversion from Z-matrix to cartesian coordinates failed' in Gaussian09. Any suggestions?

I'm doing my calculations using G09W. My OS is Windows 7 (64-bit). This error occurs in the frequency calculation. What's even weirder is that it seems to happen right after everything is already ...
6
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1answer
30 views

I get this error in Gaussian09 even though I have enough memory. “Error termination in NtrErr: NtrErr called from FIOCnC.” Any suggestions?

Whenever read-write files get to about 20Gb, I get this error. I know that, in G09, one rw file is limited to 16Gb. So I have set the software to write several RWFs. But it still keeps producing this ...
10
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1answer
68 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 ...
9
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1answer
42 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)...
7
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1answer
61 views

Bond Order: When and how is it used today?

We all learned about single vs double vs triple bonds in high school chemistry and biology. We learned that unsaturated fats have double bonds whereas saturated fats have only single bonds, which ...
5
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1answer
86 views

How is MCTDH derived?

In the multi-configurational time-dependent Hartree (MCTDH) method, the equation of motion is derived from the time-dependent Schrödinger equation by substituting the wavefunction ansatz expanded in a ...
13
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2answers
126 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 ...
31
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1answer
1k 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,...
14
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1answer
179 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 ...
11
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2answers
272 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-...
8
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1answer
97 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 ...
15
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2answers
306 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, ...
12
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1answer
283 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 ...
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 ...
19
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2answers
297 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 ...
19
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5answers
308 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
132 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 ...
13
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1answer
65 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 ...
25
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1answer
273 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 ...