Questions tagged [quantum-chemistry]

Quantum chemistry is a subfield of quantum mechanics. Like its parent field, quantum chemistry focuses on understanding physical phenomena occuring at the atomic scale. Quantum chemistry however is more focused on providing useful descriptions of electronic structure to aid in understanding chemical problems (e.g. reactions, spectra, dynamics, ...).

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Complexity in computing the integrals involving higher angular momentum

I am quite familiar with Hartree Fock theory. However, writing it into program is an entirely different task, and that is where I have a couple of questions. I wrote a Hartree-Fock routine in Python ...
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Why is it possible to image LUMO if these orbitals are, by definition, unoccupied?

LUMO stands for lowest unoccupied molecular orbital. If it's unoccupied, how is it possible to image it? See for example in this paper ("Pentacene imaged with STM and NC-AFM").
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Is there a general / simple formula for Slater's rules?

I just learned how Slater's rules work on Wikipedia. These rules really are very simple. But the presentation of the rules seemed not very efficient. I would think there would be someway to set up and ...
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Coefficients and Parameters for contracted Gaussian basis sets

I'm trying to write a program to calculate fixed-point Hartree-Fock level energies of molecules (for my amusement) and everything makes sense but this. I've been agonizing over this for almost 3 hours ...
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Is it reasonable to use natural orbitals of an unrestricted HF calculation as a basis to start a CASSCF calculation for a radical

Let's say we want to run a CASSCF calculation of a radical, for example the cation of a neutral closed-shell molecule (therefore, we calculate an open-shell radical cation). MOLCAS, as well as ...
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Why does the exchange interaction in Hartree-Fock theory lower the total energy?

In Hartree-Fock theory, the expectation value of the total energy can be written as $$E = \langle\Psi| H |\Psi\rangle = \sum_{a} \langle a| h |a \rangle + \frac{1}{2}\sum_{ab} \big( [aa |bb] - [ab|ba]...
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What function is used to graph the potential curve of two hydrogens with parallel spins?

TL; DR I am asking for an analytic potential curve function for the triplet state of $\ce{H2}$ (preferred) with experimental parameters, or experimental data (enough points to realise a smooth-...
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Functional variation in derivation of the Hartree-Fock equations

In Szabo and Ostlund's Modern Quantum Chemistry, the procedure of single determinant energy minimization is presented. Omitting the whole procedure, I have a question about functional variation during ...
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Can single molecules of C and O2 react in isolation, and if so how will momentum be conserved?

I am trying to figure out how is it possible to reconcile the reality of exothermic reactions, which means that kinetic energy is transferred to (heats) the surrounding matter, with the principle of ...
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Quantum mechanical explanation for Le Chatelier's principle?

We know from basic chemistry that a reaction reaches equilibrium as described by Le Châtelier's principle. What's the quantum mechanical explanation to this principle? How a change in temperature/...
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Mathematics in inorganic and quantum chemistry

Is mathematics important to learn concepts in inorganic chemistry, such as point groups, point symmetry, because doing these topics requires one to determine the symmetry of the elements and to ...
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Most probable point for finding an electron in the 1s orbital of a Hydrogen atom

My professor says that the most probable point for finding an electron in a 1s orbital of a hydrogen atom is at its origin. He explains this by citing the fact that the square of the wave function ...
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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 ...
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What is the meaning of “no Hessian Eigenvalue ”?

When running GAMESS for finding Hessian Eigenvalues and frequencies ( using Optimization with hessian calculation at the end ) , I have seen there are times that it comes out "without any frequencies"....
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Why are these molecular orbitals invalid for hexatriene?

Here are the MOs (more accurately, the breakdown of the MOs) for conjugated hexatriene: Now, when first asked to draw the MOs myself, I drew this one for $\psi_3$: It still has two nodes, and is ...
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Transforming AO basis electron repulsion integrals into molecular spin orbital basis?

I am attempting to do project 5 on Daniel Crawford's website. I'm having some trouble reproducing the MP2 energy, which is given in project 4. In project 4, the MP2 energy is given as: $$ E_{MP2} = ...
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How to simulate chemical reactions in GAMESS?

I am rather newb in GAMESS field but finally, learned to make input files for most molecular simulations and could even use TD-DFT for excited states (special thanks to Geoff Hutchison for his help). ...
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The “rules” for LCAOs in Molecular Orbital Theory

In our course on physical chemistry, which involves MOT, we have been taught that in the LCAO approach, the wave function for a molecule … say hydrogen ion ($\ce{H2+}$), can be approximated by a ...
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Why Acetone does not behave like its computational values?

I am trying to simulate the excitation state of acetone. I ran TDDFT for it both in gas phase and solvated state in water (both implicit and explicit water). The experimental data say that acetone ...
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How to make approximation of rotational partition function of diatomic linear molecules?

Using the rigid rotor approximation to the level energies, and such other appropriate assumptions, we can approximate rotational partition function, $Q_{\mathrm{rot}}$, of linear molecules as follows: ...
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How does absorption spectroscopy work?

So a beam of white-light photons passes through a gas and various photons, having exactly the same energy as the gas's molecular energy transition state deltas, get absorbed. We then get a ...
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Better functionals to work with transition metals

My Masters in inorganic chemistry is focused in magnetic properties of transition metals; great part of the work is done with DFT methods using softwares like ORCA and NWChem, but since my University ...
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Molecular orbitals used with CCSD(T) geometries

Suppose I am given some geometry data (say, of a water dimer) from CCSD(T). If I were to do a single-point energy calculation to generate orbitals for the system, wouldn't CCSD give something more ...
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Do multielectron atoms have nodes?

For hydrogen, other than in the 1s state, the electron wavefunctions have radial and/or angular nodes where the electron probability density is zero. In helium or further atoms with more than one ...
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Why is the orbital angular momentum of a pi electron along the axis of two atoms' molecule one?

I'm reading quantum chemistry. The book says that the orbital angular momentum of a $\pi$ electron along the symmetry axis of a molecule made up of two atoms is $\pm 1$. I think this is a primary ...
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Importance of higher order exchange terms?

The MP2 ground state energy of a molecule or solid can be written as $$E^{(2)} = \frac{1}{2}\sum^{\text{occ.}}_{ij}\sum^{\text{virt.}}_{ab} \frac{\langle ij | ab \rangle [\langle ab|ij \rangle - \...
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Can pi backdonation occur on non-metal centers?

The common example of back-donation is the interaction of a CO molecule with a metal center (d-orbitals) on a surface. Can a similar mechanism occur between CO and a non-metal center, like oxygen on ...
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Rotation around CC bond in 1,2-dioxetane: transition states and local minimum structures

TLDR: What: Potential energy surface associated with rotation around CC bond in the ring-opened version of 1,2-dioxetane. How: State average(4)-CASSCF(12,10)/VTZP constrained geometry optimizations ...
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Visualizing atoms in XYZ file with colors representing partial charges

I have a file containing the atomic coordinates (in XYZ format) of a structure, and I also have a list of the partial atomic charges for each atom. I'd like to create an image of the structure where ...
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Is the notion of orbitals different in theoretical chemistry?

I am just an average chemistry student that is currently exposed to theoretical chemistry. And I am seeing very puzzling things, such as 1p, 2f, 3g orbitals. In my attempt to reconcile this, I was ...
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Significance of 4π in the uncertainty principle

What is the significance of the quantity 4π in the Heisenberg uncertainty principle? $Δp \times Δx = \frac{h}{4π}$ The focus of the question is to learn why the ratio of a circle's circumference to ...
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Defining and testing custom made DFT functionals?

What is a good and free software for this endeavor? Any recommended articles on the subject? I want to design dft functionals, trying out different parametrizations and optimizing mixing constants for ...
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What is the difference between ψ, |ψ|², radial probability, and radial distribution of electrons?

I am very confused regarding these four terms- ψ,|ψ|², radial probability and radial distribution I know that ψ is called the wave function but is it the same as radial probability? I also know ...
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The energy gap between a pi-conjugated system with (2 bonding and 1 anti-bonding orbital) and (1 bonding and 2 anti-bonding) orbitals

I asked a question previously about "why" it is the case the expanding the size of pi-conjugated systems decreases the required energy to excite an electron from a HOMO to a LUMO band: Why does the ...
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477 views

Negative Kelvin Temperature

I remember my Physical Chemistry Professor saying that very tiny negative Kelvin temperatures have been achieved on the quantum level. Is this true?
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How would you know if the light emitted is of one wavelength or if it is a mixture of two or more wavelengths? [closed]

I am just learning some chemistry after a long time without studying it and I have thought about something. Some copper compounds emit green light when heated to flame. So, I would like to know if it ...
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Why is the Maxwell–Boltzmann formula inapplicable at low temperatures?

This Wikipedia article states that at sufficiently low temperatures (or high pressure), quantum effects lead to a breakdown of Maxwell–Boltzmann statistics. I know that the spacing of energy levels ...
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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 ...
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Why search new elements?

I've read this today. From the little I know, they discover new elements by bombarding some atoms in particle accelerators (I guess) and these new elements last a few miliseconds. So what's the ...
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Why are excited Slater determinants used to describe electron correlation?

Usually the first step in describing the electronic wave function of a molecule or atom is to describe it with a single Slater determinant. I get that this is an independent particle approximation, ...
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992 views

Irreducible representations and system states connection

Let's say I have the molecule $\ce{N2+}$. Its symmetry point group is clearly $D_{\infty\mathrm h}$. But I'm confused by its irreducible representations. I know there are 8 of them, but as I ...
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714 views

Hartree-Fock dipole moment

I read this sentence in a paper (J. Phys. Chem. A 2014, 118 (20), 3678–3687. DOI: 10.1021/jp502475e): Calculations using Hartree–Fock theory systematically overestimated dipole moments and ...
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833 views

Matching a Slater-type wavefunction with a minimal (STO-nG) Gaussian basis set

I have on hand a high-precision wavefunction expressed in Slater orbitals. I need to express it as accurately as possible in a minimal Gaussian basis set. For background: I am currently working with ...
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Use of basis set in DFT (Density Functional Theory)

Basis sets are used to guess the electronic wave functions for Hartree Fock or similar methods, which are quite legitimate since these methods deal with the wave function of each and every electron. ...
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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}...
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763 views

Usefulness of the outdated Bohr model?

Whilst the Bohr model is incomplete and incorrect, it had limited usage in predicting spectral lines. In the same way, could it possibly with limited accuracy, be used to predict the outcome of ...
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Why are so many wave functions associated with hydrogen?

According to Wikipedia, there's an infinite set of possible wavefunctions (orbitals) for the hydrogen atom: $$\psi_{n\ell m}(r,\theta,\phi) = \sqrt {{\left ( \frac{2}{n a_0} \right )}^3\frac{(n-\ell-...
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What are the applications of quantum field theory to chemistry?

I have a physics background and I learned how quantum field theory is applicable to condensed matter physics, and since some parts of condensed matter physics overlap with chemistry I was wondering if ...
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Is the d-block ever part of the valence electrons?

The d-block sub-shell is always one shell lower than the s-block and p-block sub-shells. Assuming only the electrons in the highest energy shells count toward the set of valence electrons (is that ...
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How can enzyme/substrate reactions that adhere (largely) to quantum theory also require 'Newtonian' consideration of gravity?

I'd just like to ask for a little clarification here due to confusion from interdisciplinary studies. I'm currently reading the 1976 paper related to the recent 2013 Nobel Prize for Chemistry, by 2 ...

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