Quantum chemistry is the subfield of quantum mechanics. Like the parent it focuses on physical phenomenon that occur at an atomic scale, usually specifically aiming at chemical reactions.

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Unequal ionization energies of methane

Why does methane have two different ionization potentials? How does this work? I understand that MO theory predicts C-H bonds of differing strength, while hybridization predicts C-H bonds of varying ...
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What is the point of introducing virtual orbitals in Hartree-Fock calculations?

Let us for simplicity discuss RHF formalism. For $2n$-electron system we have $n$ Hartree-Fock equations written for $n$ spatial orbitals $\{ \phi_{k} \}_{k=1}^{n}$ $$ ...
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How can two orbitals constructively and destructively interfere simultaneously?

The molecular orbital theory dictates that when two atomic orbitals form molecular orbitals, then two molecular orbitals must form (i.e number of atomic orbitals = number of molecular orbitals). For ...
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How do electrons travel through nodes

I understand this is a basic question, but I'm having such a hard time wrapping my head around it. I'm trying to avoid thinking about it as an actual "particle" but as a wave, but that confuses me ...
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What is the Quantum Number for the last electron of V+ ion? [closed]

What will be the quantum number of the last electron of $\ce{V+}$ ion? Is it from 3d or from 4s? Which electron should be considered the 'last electron' in $\ce{V+}$ ion?
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52 views

How much energy does it cost to have electron configurations that are not in accordance with Hund's rules?

What is a ballpark figure for the difference in energy for an atom that follows Hund's rule vs one that has two electrons with opposite spins? I'd be interested to know carbon and nitrogen. Is there ...
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55 views

What are the Waves Modeling when Referring to the Atomic Orbitals

It is taught that the orbital shapes derive from wave functions with different numbers of nodes. For example, the "s" orbital comes from a wave that has one node. But what are the waves modeling? A ...
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32 views

Can electrons switch orbitals within a shell?

I know that electrons can move from say 2s orbital to an unoccupied 2p orbital, as in Carbon atom which can form 4 bonds this way. But I want to know is it possible for an electron say in orbital 2p ...
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45 views

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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80 views

Energetic Placement of Atomic Orbitals in the HCl Molecular Orbital Diagram

How are the 3p orbitals of chlorine lower in energy than the 1s orbital of hydrogen?
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59 views

Two-electron Integrals over Gaussian Plane Waves [closed]

Is there an efficient method to compute the two-electron integrals over the basis set represented by a product of a Gaussian function $g(r)$ and plane wave: $\psi(r)=g(r)e^{ikr}$ where $\lambda=1/k$ ...
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209 views

Are there any full worked examples of DFT calculations?

I just started learning DFT and now I am totally confused. Assuming I want to use B3LYP: \begin{align} v_s\left(\textbf{r}\right) &= v_\text{ext}\left(\textbf{r}\right) + \int d^3r\frac{n\left( ...
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78 views

What Quantum Chemistry Model Should I Use?

Sorry for such a elementary question, My Main purpose is using Quantum Chemistry for simulation of some reactions. Actually we are working on making some new polymers and I need to simulate some ...
3
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35 views

how are the frequencies at a local maximum of PES like?

On the potential energy surface, if you find a local maximum and calculate its frequencies in Gaussian or something like that, will you get all negative frequencies or all positive frequencies? I know ...
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30 views

Calculate Excited States using Gamess

I am trying to compute excited states and the PES of a O2 molecule using GAMESS. I have, however, not yet succeeded even for a simple molecule such as H2. Does anyone know where to find a good ...
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131 views

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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38 views

Number of orbitals in full configuration interaction

I am using one quantum chemistry package where I am supposed to assign a number of active orbitals I want to be included in the FCI (full configuration interaction) calculation. For example if I have ...
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2answers
56 views

I am trying to picture how electrons move around in atomic orbitals

Are they thought to continuously pop in and out of existence at various points inside the orbital defined by probabilities or do they follow definite paths that are made fuzzy by the Heisenberg ...
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135 views

What's the difference between PBE and B3LYP methods?

I can't find an answer to that question. I was told that in B3LYP, more variables implemented in the method are empirical, but I can't find anywhere if it's true, and I'm sure it's not the only ...
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68 views

Semi-Infinite Potential Square Well: Negative Potential

I am having trouble with a problem involving a Semi-infinite potential square well: I have written down some notes that I added to the post. I have problems understanding the physical situation. See ...
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1answer
46 views

How to calculate the rotational constants for different states from a stick spectrum?

Given a stick spectrum for an unknown diatomic molecule, how do you determine the rotational constants $B_\nu$ for both the excited and ground states given that $$E_{j}=B_{\nu}j(j+1)$$
3
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1answer
38 views

Probability of measuring a particle in the ground state: having trouble with the integration

I am working on a problem in the context of an infinite symmetric potential barrier around the origin, with barriers at $x=- \frac{a}{2}$ and $x=\frac{a}{2}$. The wave function is a symmetric ...
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2answers
75 views

Normalizing the wave function $\psi (x) = A$

I was trying to normalize the wave function $$ \psi (x) = \begin{cases} 0 & x<-b \\ A & -b \leq x \leq 3b \\ 0 & x>3b \end{cases} $$ This is done simply by evaluating $$ ...
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50 views

Calculating Commutator of Differential Angular Momentum

First, I realise this may not strictly quality as chemistry, but it is part of a quantum chemistry/quantum physics course, so I hope it goes through! I have tried for hours to calculate the ...
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Why is the groundstate electronic degeneracy of monoatomic O(g) 5?

A problem in my Pchem textbook states that the electronic degeneracy of an oxygen atom is 5. My reasoning leads me to believe that the degeneracy is actually 6 (S = 1, L = 1, so (S+1)(L+1) = 6. This ...
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Why is a singlet state called singlet and a triplet state called triplet?

I kind of get the idea of singlet and triplet states. But why are they called singlet and triplet (what is the single and what is the triple in these cases)? I feel that I am missing something ...
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115 views

What is the appropriate procedure for determining the quantum numbers of an electron?

How do I find the set of quantum numbers for a specific electron in an element? For example, Calculate the set of quantum numbers for the 19th electron in chromium. The electronic configuration ...
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82 views

Why is the 8-electron rule more important than the 2- or 18-electron rule

Why is the fulfilled electronic configuration of only $p$ orbital is stable. I mean why $II-B $ group with fulfilled $d$ orbital,$II-A$ group with fulfilled $s$ orbital...are not stable. Why makes ...
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73 views

Degenerate Orbitals

How do I know if an atom has degenerate orbitals? My Understanding I understand that degenerate orbitals mean orbitals that have the same energy level for the same n. However, how do I distinguish ...
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62 views

Bonding, multiplicity and quantum chemistry for FeP$_2$

I'm trying to do some quantum chemical calculations for the linear molecule FeP$_2$ in the gas phase as well as its crystal (orthorhombic symmetry like here). I am lead to believe that in both cases ...
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75 views

Are multiple transition states possible? Criteria to choose the correct one?

I am studying a reaction using electronic structure methods using various software packages (ORCA,GAMESS, G09) and have found two possible transition structures between product and reactant. Both have ...
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134 views

Which of the following statements is not true about quantum theory? [closed]

Which of the following statements is not true of quantum theory: A: Light energy from an object is emitted in small packets called quanta. B: The Bohr atom is quantized with the electrons ...
3
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146 views

What does the subscript of atomic orbital mean?

As everyone knows, the atomic orbital can be classified as $s, p_z, p_x, p_y, d_{z^2},d_{xz},d_{yz},d_{xy},d_{x^2-y^2}$ and so on. I want to know the meaning of $z^2,x^2-y^2$ and so on. Maybe this is ...
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234 views

Does partial charge violate the law of quantization of charge?

Well, we know that charge is quantized. Quoting wikipedia below, Charge quantization is the principle that the charge of any object is an integer multiple of the elementary charge. And we know ...
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92 views

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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991 views

Why is the alpha particle called a particle when it is made of four particles?

We know the alpha particle is the nucleus of helium. It contains four subatomic particles - two protons and two neutrons. The protons and neutrons are further made of particles called up and down ...
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559 views

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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3answers
519 views

Why does the energy gap for π - π* transitions shrink with the size of the pi-conjugated system?

Quoting from this site: As conjugated pi systems become larger, the energy gap for a π - π* transition becomes increasingly narrow, and the wavelength of light absorbed correspondingly becomes ...
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230 views

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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What is Inert Pair effect?

I was reading the p-block elements and found this fancy thing used everywhere in this topic. But the book does not explain it very well. So, What is Inert Pair effect? Give full explanation and also ...
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How to properly denote imaginary magnitudes of vibrational frequencies

Vibrational frequency computations provide the magnitudes of vibrational frequencies for each vibrational mode of a molecule ($3N-6$ degrees of freedom for non-linear molecules and $3N-5$ for linear ...
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Quantum Chemistry: For what value of n (principal quantum number) will the expectation value of the radius equal to one-half of a 100 Å box size? [closed]

A hydrogen atom is in cubical box with side lengths equal to 100 Å. For what value of n (principal quantum number) will the expectation value of the radius equal to one-half the box size?
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113 views

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 ...
4
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1answer
225 views

Suggest methods and basis sets for a variety of systems

Please help me with any/all of the cases below. In the following cases, the named method and basis set are not suitable for the chemical systems. Why aren't they? Could you suggest a suitable ...
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Do these compounds Na$_x$IrO$_2$ and NaOsO$_2$ exist?

Based on some physics background, I want to know whether have we synthesized these two materials: $\ce{Na_{x}IrO2}$ and $\ce{NaOsO2}$ in the lab, if so, what properties do they have? I will be very ...
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Quantum chemistry: CCSD(T) with aug-cc-pVTZ on omeprazol

I know that CCSD(T) with aug-cc-pVTZ basis set for geometry optimization of omeprazol, i.e. the active substance in Losec is not suitable using the normal computers. The substance is: omprazol Can ...
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570 views

Why are wave functions orthogonal?

It seems that all wave functions studied in physical chemistry are orthogonal (e.g. particle in a box, hydrogen atomic orbitals). Does this come about because we purposefully make them orthogonal, or ...
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119 views

Bonding and nodes with molecular orbitals

If there is more overlap of the same sign wave function, this apparently leads to less energy. However, I don't understand why. My professor said that there would be less nodes, and less nodes means ...
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69 views

An alternative basis set for analytical computation of two electron integrals

It is well known that the usage of the Gaussian basis set, in contrast to Slater basis set, leads relative simple semi-analytical expressions for the two electron repulsion integral $(ab|cd)$. Could ...
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Magnetic moment of coordination complexes?

To understand the commonly quoted magnetic values of coordination complexes (central ion) we use $$m_l=\sqrt{n(n+2)} \text{BM where BM}=\frac{e\hbar}{2m_e}\text{JT}^{-1}$$ $n$=number of unpaired ...