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, ...).

Filter by
Sorted by
Tagged with
0
votes
0answers
18 views

Which is more stable and why: a half-filled s subshell or a half-filled p subshell?

Is $\mathrm{(1s)^2 (2s)^2 (2p)^6 (3s)^1}$ or $\mathrm{(1s)^2 (2s)^2 (2p)^3}$ more stable? Which has larger ionization enthalpy?
-1
votes
0answers
12 views

Two electron integrals between 1s STO

I am trying to understand SCF cycle by trying to code up solved example from Quantum Chemistry by Levine (page 443, 5th edition). The problem is shown below: One electron integrals are straight ...
0
votes
0answers
28 views

Obtaining energy and wave function for excited states

Suppose I have done a HF calculation and obtain the energy and wave function of ground state. If I want to produce the spectrum of the specie (say Hydrogen molecule), should I use CI (configuration ...
0
votes
1answer
44 views

Do quantum particles have mass? [closed]

Quantum particles are never objects but are always waves. But do they have mass or can they carry mass?
2
votes
0answers
42 views

The true shape of p orbitals

I'm trying to model $\mathrm{p}$ orbitals using a 3D program. What confuses me is that there are 2 shapes of $\mathrm{p}$ orbitals. I'm not sure which one is the accurate model. And what is the ...
2
votes
0answers
38 views

How is the ΔL = ±1 selection rule derived?

I understand there are selection rules in spectroscopy and quantum mechanics which governs whether a transition happens or not. For example, in atomic spectroscopy, the total orbital angular momentum ...
3
votes
0answers
18 views

Associated occupation numbers of frozen natural orbitals (FNOs) from MP2 method

The paper Frozen Virtual Natural Orbitals for Coupled-Cluster Linear-Response Theory gives the following (simplified) description of frozen natural orbitals (FNOs). In the MP2 method, the unrelaxed ...
-1
votes
1answer
45 views

Can carbon have electrons in a 5d orbital? [closed]

(a) Sketch the radial probability distribution for a 5d orbital in a carbon atom. You should label the axes, but do not need to include numbers. Use arrows to indicate the radial nodes. (b) ...
-2
votes
0answers
34 views

How to solve this error in Gaussian09? UNABLE TO DETERMINE LAMDA IN FmD114 [closed]

I'm doing CCSD(T) and CCSD calculations on a series of small molecules. Several calculations terminated without any problems but one CCSD(T) calculation and one utilizing CCSD gave me this error after ...
5
votes
0answers
43 views
+50

On the localized screening of cerium (when compared to iron)

I was reading the paper 'A linear response approach to the calculation of the effective interaction parameters in the LDA+U method' by Cococcioni - (Paper here) . Quoting from the first paragraph on ...
5
votes
2answers
71 views

High-temperature DFT

Why is it so difficult to perform DFT calculations that consider temperature? I have seen that time-dependent DFT is needed to model systems at high temperature. Why is this the case? What about ...
2
votes
1answer
37 views

What is the meaning of the J value in term symbols [closed]

I understand how to work out the J values of term symbols. e.g. a d1 configuration gives J values of 5/2 and 3/2. resulting in 2D 3/2 However I dont understand the meaning of it, and why it arises.
4
votes
0answers
32 views

Is there a mistake in diagramatic representation of RS perturbation theory described in Szabo and Ostlund textbook?

I'm trying to understand rules of the diagrammatic representation of the many body perturbation theory that described in Szabo and Ostlund "Modern Quantum Chemistry" textbook. One of the first rules ...
2
votes
1answer
118 views

The particle in a two-dimensional box model and graphene

I have been having trouble with understanding and solving the following problem. For two-dimensional conjugated systems, we may use the particle in a two-dimensional box model. In this case the ...
2
votes
0answers
42 views

What is the precise meaning of “in-phase” (and “out-of-phase”) in this context?

I am currently studying the textbook Infrared and Raman Spectroscopy, 2nd edition, by Peter Larkin. In a section entitled Symmetry: Infrared and Raman Active Vibrations, the author says the following: ...
3
votes
0answers
25 views

Determining symmetry correction by looking at rotational quantum states

I'm trying to understand the relationship between symmetry correction and rotational quantum states, particularly in the case of dipoles with identical atoms. For an angular momentum quantum number $...
-1
votes
0answers
17 views

Racah's seniority number

Racah's seniority number $\nu$ distinguish two equal $LS$ state,for in example in $d^3$ we have two $^2D$ states so we give them the seniority number number $\nu=1$ and $\nu=3$ distinguish them. My ...
-1
votes
1answer
41 views

How to find the number of valence electrons with the help pf quantum numbers? [closed]

I'm bit confused on how to find the number of valence electrons. For example, Boron ($\ce{B) has the electronic configuration as: $\mathrm{1s^2 2s^2 2p^1}$. So what would be its number of valence ...
4
votes
0answers
49 views

Symmetry Adapted molecular orbitals of Formaldehyde

I am currently trying to find the Symmetry Adapted linear Combinations (SALCs) of Formaldehyde $\ce{H2C=O}$, and use the 1s orbitals on the hydrogen atoms, and 2s, 2px, 2py and 2pz orbitals on the ...
0
votes
2answers
50 views

Why is atomic orbital one electron wave function? Why distance from centre is proportional to angular wave function?

Consider the following statements: An atomic orbital is one electron wave function $\psi(r,\theta,\varphi)$ obtained from the solution to the Schrödinger equation. There two electrons in an atomic ...
4
votes
1answer
83 views

Quantization of particle on a sphere

While studying particle on a sphere, I read that boundary condition on $\theta$ and $\phi$ results in quantization. I understand boundary condition for $\phi$ part which varies from 0 to 2$\pi$. As $\...
1
vote
2answers
46 views

What is the relationship between entropy and heat capacity?

The molar heat capacity of hydrogen gas and deuterium gas are nearly the same, 28.8 J/K mol and 29.2 J/K mol respectively, but the absolute entropy of deuterium (145.0 J/K mol) is significantly larger ...
0
votes
1answer
31 views

Wave function of a perturbed system

In perturbation theory, wave function of a perturbed system can be expressed in a power series of some $\lambda$ as $\psi^{(0)} + \lambda\psi^{(1)}+\lambda^2\psi^{(2)}......$. Then we express ...
1
vote
0answers
47 views

How do you optimize an organo-metalic compound containing cerium in in Gaussian?

I am trying to perform optimisation of the molecule attached. I need to get partial charges and frequencies. The issue is that it doesn't converge with following input: ...
1
vote
0answers
17 views

Would the 1st excited state of Li2 be vibrationally excited?

I'm quite torn about an interesting past paper question that's come up during my revision, shown below. On the one hand, you're promoting an electron in a bonding orbital. In general, since this ...
4
votes
1answer
152 views

How do you calculate the norm of a function describing a molecular orbital?

I'm a bit confused by a past paper question I came across for one of my courses, shown below. The wavefunctions corresponding to the lowest, $\psi_{1}$, and second-lowest, $\psi_{2}$, energy ...
1
vote
0answers
44 views

Wave function of a harmonic oscillator

In a question it is given that consider $H-X$ chemical bond and find $H-X$ bond distance for which there is zero probability density of finding the proton. It is easy to solve we have just to ...
8
votes
1answer
61 views

Ground state term symbol for Cerium (Ce)

I need to calculate the term symbol of the ground state configuration of Cerium. The electron configuration is: $[Xe] 4f^1 5d^1 6s^2$ Since the d and f electrons are in different subshelles, they are ...
3
votes
1answer
56 views

I'd like to model some compounds with DFTB, but some elements don't have parameter files in the official site. Is there a workaround?

I was reading a bit about this technique, Density Functional based Tight binding (DFTB). It's claimed it can provide a speed improvement of several fold when compared to plain DFT, still giving ...
5
votes
2answers
111 views

Oscillation of particle in harmonic oscillator

In harmonic oscillator, as vibration quantum number increases then does the distance of oscillation of particle from the mean position where there is minimum potential energy increases? If yes, then ...
0
votes
0answers
32 views

Finding vibrational frequencies and heat capacity from bond enthalpies?

So I have heat of formation data for H2, HD, and D2, and I'm being asked to find the difference in vibrational frequency and zero point energies between H-H and H-D. I know that i need to use the ...
5
votes
1answer
155 views

Does an orbital have its own energy, separate from the electron? Can I picture them as a entity independent from the hosted particle? [closed]

Does an orbital have its own energy, separate from the electron? If so, is it possible for an electron to have a different energy than the orbital it occupies?
5
votes
2answers
81 views

Particle in a box with different barriers between two molecules

I'm trying to understand different ways to solve a PIB model. I'm given a molecule with a tetrahedral geometry and there are two related ground states - electron pair pointing up or down. I'm being ...
4
votes
1answer
64 views

Overlap integrals in Hückel theory

In Hückel theory we are only interested in π systems, where $\mathrm p_z$ orbitals overlap. One of the approximations in Hückel theory is that the overlapping $\mathrm p_z$ orbitals are orthonormal: $...
3
votes
1answer
126 views

How do we obtain the π molecular orbitals for allene via Hückel theory?

For a conjugated system, for instance 1,3-butadiene, we can try to obtain the π molecular orbital energies and wavefunctions via Hückel theory. In our given molecule, there are 4 interacting Pz ...
-1
votes
1answer
50 views

How is it that the Inner Periodic Table Sections (have 15 elements each) versus their f-block (with only 14 element’s electrons each)? [closed]

Given: 1. the “Lanthanide and Actinide” Inner Periodic Table Sections (each have 15 elements) versus the f-block Orbitals (which only have a capacity for 14 out of the 15 element’s electrons each) - ...
4
votes
1answer
51 views

Non-existence of time operator in quantum mechanics

In quantum mechanics, there is no operator for time instead of this we we have $\psi(t)$ which is related to $\psi(0)$. In a book, I read that if there is an operator for time, then it will not be ...
0
votes
0answers
32 views

Probability density of a particle in a 2D box with given coordinates

Wavefunction of particle in a $2$D box with sides $L$ in the $x$ and $y$-axis: $$φ = \sqrt{\frac {4}{L^2}} \sin\frac{n_1πx}{L}\sin\frac{n_2πy}{L}$$ For the different states ($n_1=1$, $n_2=2$), (...
2
votes
0answers
42 views

What are the alternatives we have to the VEDA 4 vibrational energy distribution analysis software?

Said software is very useful when you need to assign the peaks in vibrational spectra to different vibrational modes found in a hessian calculation for a given molecule, as described in Jamróz, Michał ...
0
votes
0answers
15 views

Wave function characteristics

In a book, I read that the wave function of a system should not become infinite in a finite region and also should vanish at infinity. For the first part I think that if wave function is infinite in ...
2
votes
0answers
26 views

Why is the transition (0,0,0) -> (1,0,1) observed in a gas phase IR spectrum of CO2?

Let ($v_1$,$v_2$,$v_3$) denote the vibrational state of $CO_2$. Why is the transition $(0,0,0)\rightarrow (1,0,1)$ observed when the trasition $(0,0,0)\rightarrow(1,0,0)$ (asymmetric stretch) is not ...
2
votes
1answer
47 views

How to write a wave function for infinite potential well with different width than from 0 to a?

I am working on the perturbation theory problem with the infinite potential well. However, I am not sure how to write a wavefunction for different than the infinite potential well from $0$ to $a$ and ...
3
votes
1answer
39 views

Order of filling electrons in Jahn-Teller energy diagram

When filling electrons in atomic or molecular orbitals we fill first the lower energy orbitals and then the higher, according to Pauli and Hund's rule. But this seem not to be the case when ...
3
votes
0answers
146 views

Generating hybrid atomic orbitals for Boron

My professor was showing us how to form hybrid atomic orbitals for Boron by taking a linear combination of rotated 2p orbitals and 2s orbitals (of Boron). This was done in a particular manner such ...
10
votes
4answers
176 views

Molecule or molecular ion that spontaneously breaks up in two different ways

I teach physics, and I'm looking for simple, easy examples from chemistry to use in order to illustrate basic ideas about quantum mechanics for my students. A conceptually simple example is the $\text{...
0
votes
1answer
44 views

Expectation value of an observable in quantum system

In quantum systems, sometimes if the wave function is not an eigen value of an operator then it is expanded in terms of bases states, then we apply the operator over the bases states and we get ...
0
votes
1answer
35 views

What's the definition for mole before Avogadro's constant?

I have been searching for about 3 days how Avogadro's constant is found, and as all of us know the value is determined by division of charge of one mole of electrons (Faraday's constant) by the charge ...
6
votes
2answers
86 views

Computing energy of Guassian basis set

Actually, I'd like to compute decay rate using : $$ \Gamma = 2\pi \ | \langle ab|V|cd \rangle|^2 \delta\ (\epsilon_f-\epsilon_i) $$ where a, b, c, d are the MOs and V is coulomb interaction, and $\...
0
votes
0answers
31 views

Trouble understanding equation from density functional theory

In the review by Oliviera et. al., I do not understand how the authors get equation 14: \begin{equation} E[\rho]=\sum_i^Mn_i\langle\Psi_i|-\frac{1}{2}\nabla^2+v_{ext}(\vec{r})+\frac{1}{2}\int\frac{\...
1
vote
5answers
157 views

Why is the molecular structure of water bent?

Water has the formula $\ce{H2O}$ and we can draw a Lewis structure with two lone pairs on the central oxygen. As a physics student and not a chemist, I think to myself, "Okay, there are two lone pairs,...

1
2 3 4 5
23