Questions tagged [ab-initio]

For questions relating to quantum chemistry using wavefunction-based methods and solving the Schrödinger equation.

Filter by
Sorted by
Tagged with
2
votes
2answers
54 views

Effective core potential and Gaussian basis set

Is using the effective core potential (ECP) and Gaussian basis set simultaneously in ab initio calculation a valid approach? For example, is it correct to use ECP for one of the atoms and, instead of ...
2
votes
0answers
31 views

Potential energy surface in transition state theory

I would like to study the typical reaction from the transition state theory: $$\ce{AB + C <=> ABC^‡ -> A + BC}$$ Assuming $\ce{ABC^‡}$ is a colinear molecule, the potential energy surface of ...
5
votes
0answers
27 views

Is the Hartree-Fock determinant always the minimum energy single Slater determinant solution to a molecular Hamiltonian?

Is it always true that the Hartree-Fock slater determinant is the minimum energy single slater determinant solution to a given molecular Hamiltonian $H$? By construction, any determinant $|\chi_{i_1}\...
0
votes
0answers
52 views

How to plot the wave function after a Hartree-Fock calculation?

I have written a code in Mathematica and have obtained the final energy of the hydrogen molecule. I have used Gaussian basis functions for this calculation. Now I need to plot the ground state wave ...
6
votes
1answer
227 views

Ab initio method to calculate C-C bond dissociation energy?

I'm trying to review how to calculate bond dissociation energy for C-C in ethane, a very simple calculation (or should be). I get the following Hartree energies for SCF 6-31G(p) for methane, ethane, ...
6
votes
1answer
102 views

Is there a rule of thumb to predict when coupled cluster might dip below the variational limit?

Coupled cluster is a non-variational method, meaning that it can give energies that are below the true FCI energy (the "variational limit" for variational methods). Often coupled cluster ...
1
vote
1answer
178 views

What are the steps of Hartree-Fock Theory?

Outline the steps involved in the Hartree–Fock method for the calculation of molecular electronic structure. I understand the basics of HF, in that it does not account for electron correlation (...
1
vote
0answers
49 views

Calculating the Molecular Orbitals of a Molecular State [closed]

I am performing ab initio calculations on a heteronuclear diatomic compound using the MRCI method on the MOLPRO quantum chemistry package. I obtained the molecular states of the compound but I was ...
0
votes
1answer
64 views

Ab Initio and Molecular Orbitals [closed]

What is the difference between ab initio calculations and molecular orbital theory?
2
votes
0answers
32 views

Guide to DFT functionals [duplicate]

Having recently begun studying Quantum and computational chemistry in depth... I am looking for a manual, text, or guide of any form that could explain the basic formulations and potential of the ...
1
vote
2answers
466 views

Spectroscopic constants - explanation

I'm a beginner in spectroscopy and I have troubles understanding papers describing values of spectroscopic constants. This paper, for example, describes constants $T_e, R_e, \omega_e, \omega_ex_e, B_e,...
6
votes
0answers
205 views

How to evaluate the expectation value of the Hamiltonian for a three electron Slater determinant?

I'm having trouble with a few concepts dealing with slater determinants and many electron systems. In particular, when dealing with 3 electron systems, I know that $$ \psi_0 = \frac{1}{\sqrt{6}} \det ...
2
votes
0answers
39 views

tolerable error, CASSCF simulation of UV/Vis spectrum

Please forgive my ignorance of experimental spectroscopy. I am a computational chemist, not an experimental one. My question is: what would be considered a tolerable or reasonable difference between ...
2
votes
0answers
28 views

GVP(PP) orbital energies?

Hartree-Fock orbital energies can be interpreted physically, e.g. in Koopmans theorem, while one cannot assign active MCSCF orbitals to energies in general. What about GVB, say for example GVB(PP) ...
3
votes
1answer
116 views

Why is the active space for the dinitrogen cation defined this large in my example?

I have a Molpro script which computes $\ce{N2^+}$ potential energies. The significant part is the input for the wave function: ...
7
votes
1answer
570 views

Why is Coupled Cluster not variational?

It has been noted in several sources (e.g. J. Romero et al. Strategies for quantum computing molecular energies using the unitary coupled cluster ansatz. arXiv:1701.02691 [quant-ph]) that one of the ...
3
votes
0answers
109 views

Bond order analysis in molpro

I'm trying to compute bond orders in a molecule, let's consider S2 as an example. I'm not entirely sure what's the right way to do that, but I assume NBO (Natural Bond Orbital) should give me what I ...
4
votes
0answers
341 views

What all computing knowledge should a computational chemist traditionally have? [closed]

If I want to develop algorithms or computer programs for studying chemical systems, where should I start? Is learning one or two programming languages like FORTRAN enough? And if I do it, then how do ...
2
votes
0answers
47 views

Can someone suggest me important references for deriving EOM in MCSCF?

I am currently starting a project dealing with MCSCF method. Can someone please give me references (papers or lecture notes are preferred) which may be considered as the "corner stones" in the history ...
5
votes
3answers
180 views

How closely should the energy components of ab initio SCF results at a stationary nuclear configuration preserve the virial theorem?

When running a SCF geometry optimization for water molecule, I found that the energy components of the optimized geometrical structure appear to deviate from Virial theorem sightly: Kinetic energy = ...
2
votes
2answers
113 views

Potential energy surface when spin changes

I'm trying to compute potential energy surface of S4 molecule (in MOLPRO 2015) along S2-S2 bond (it has a trapezoidal shape) from equilibrium to the dissociation threshold, all the other coordinates ...
11
votes
2answers
1k views

Proof of Brillouin's theorem using the diagonality of the Fock matrix

I'm working my way through Szabo and Ostlund's Modern Quantum Chemistry. I am having trouble understanding the proof and significance of Brillouin's theorem. According to the book: Now, by ...
8
votes
1answer
118 views

What's a good measure of the Eigen-ness of a wavefunction?

Just wondering if someone can recommend an easy-to-compute measure for the Eigen-ness of a wave function, such as the wave functions computed using ab initio method? For a wave function solved using ...
12
votes
2answers
1k views

How does optimizing the molecular orbital coefficients in CASSCF improve its multi-reference capabilties?

The complete active space self-consistent field (CASSCF) method is a special kind of multi-configurational SCF procedure, attempting to combine Hartree-Fock (HF) and configuration interaction (CI). In ...
13
votes
4answers
1k views

Is coupled cluster variational for two electrons?

I know that coupled cluster (CC) is not variational for the general case. However, if we only have two electrons with one nucleus, CCSD should be exact for this system like full configuration ...
1
vote
1answer
59 views

Building a solid-liquid interface

I am trying to build a solid-liquid interface to be simulate using molecular dynamics. Since, I would be using ab initio molecular dynamics (cpmd or cp2k), I need the minimum cell dimensions. Please ...
6
votes
0answers
53 views

Paper Data Replication

I read a paper recently - Here. I found the concept of Electride materials very interesting and wanted to mess around with them myself. I tried to replicate the partial electron density plots shown in ...
9
votes
2answers
2k views

Why are correlation consistent basis sets used with DFT calculations?

Reading some papers, there is a bit of a curious observation that does not exactly make a lot of sense to me. In Quantum Chemistry, different methods exist to carry out geometry optimisation and ...
15
votes
2answers
2k views

What are typical runtimes for CASSCF calculations?

I'm doing single point energy calculations of FeS using ORCA. I originally used DFT with a variety of functionals, and these calculations all took less than a minute. Now I'm attempting to run CASSCF ...
8
votes
1answer
208 views

Ab initio effective potentials

I have been reading a journal article on ab-initio pseudo-potentials, and I need some help understanding it. The article is C. F. Melius and W. A. Goddard, III. Phys. Rev. A 1974, 10, 1528. A summary ...
10
votes
1answer
2k views

How to choose the best method and basis set for a calculation in computational chemistry?

In Quantum chemical calculations, it is not always the case that the more complex (correlated) the method or the basis set the more accurate the results. For example, it's sometimes found that MP2 ...
10
votes
1answer
1k views

Initial guess for Unrestricted Hartree-Fock calculation

I am implementing an Unrestricted Hartree-Fock (UHF) calculation, following Ref. [1]. It works fine for a odd number of electrons ($N_\alpha\neq N_\beta$) since, despite the same initial guess for $\...
11
votes
2answers
919 views

Gaussian09 vs home made Hartree-Fock implementation

I implemented a restricted Hartree-Fock (RHF) calculation in the STO-3G basis set, as described in Szabo and Ostlund's book [1]. I managed to reproduce the energies of all their calculations ($\ce{H_2}...
15
votes
3answers
3k views

Boys function for Gaussian integrals in ab-initio calculations

A few days ago I mentioned a problem with an Hartree-Fock program I am writing (HF using cartesian Pople's STO-3G basis set). I can reproduce overlap and kinetic integrals of some references for ($\ce{...
5
votes
1answer
143 views

How does one calculate chemical shift in a gauge independent manner?

The literature on ab initio calculations of chemical shift in NMR experiments usually provide Lamb's and Ramsey's formulae as the solution. Yet the expressions explicitly depend on vector potential ...
18
votes
3answers
2k views

Computing two-electron integrals with an STO-3G basis set

I am trying to implement a restricted Hartree-Fock calculation using an STO-3G basis set, for fun. I managed to perform this calculation where only $\mathrm{1s}$ orbitals are present ($\ce{H2}$ and $\...
32
votes
2answers
10k views

Is density functional theory an ab initio method?

The following comment by Wildcat made me think about whether density functional theory (DFT) can be considered an ab initio method. @Martin-マーチン, this is sort of nitpicking, but DFT (where the last ...
10
votes
1answer
529 views

What is the reason for including excited states in the wave function in post-Hartree Fock methods?

I'm new to computational chemistry, so perhaps this question has an obvious answer. I'm wondering what the intuitive reason for constructing Slater determinants that involve excited states is, when ...