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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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 ...
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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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16 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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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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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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45 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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120 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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59 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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37 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 $$E_{j}=B_{\nu}j(j+1)$$
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37 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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60 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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46 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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20 views

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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629 views

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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89 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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1answer
68 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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67 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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58 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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65 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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107 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 ...
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112 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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220 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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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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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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438 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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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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202 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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101 views

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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69 views

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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77 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 ...
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1answer
205 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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49 views

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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467 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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104 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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63 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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129 views

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 ...
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84 views

How can we write the wave function in quantum mechanics?

In quantum mechanics, while writing the wave functions we can take a product of the spatial and the spin parts.But what does it means? What is the meaning of the spatial wave function and the spin ...
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716 views

Does hydrogen peroxide exhibit intramolecular hydrogen bonding interactions?

We know that intermolecular Hydrogen bonding is seen in Hydrogen peroxide $(H_2O_2)$.But is intramolecular H-bonding,i.e. Hydrogen bond between an oxygen atom and a hydrogen atom attached to the ...
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157 views

Why are the total energies of transition states so commonly corrected for zero-point vibrational energy?

So often I see total energies of transition states corrected for zero-point vibrational energy which always confuses me. Zero point energy is the lowest energy that a ground state molecule can have ...
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126 views

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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20 views

What is the difference between ZCT Tunneling correction and Wigner Tunnelig correction in computational Quantum Chemistry?

I've been doing some computational calculations about kinect rates of a reaction and I was told to perform both ZCT and Wigner corrections to calculate these rates. But I don't know the difference ...
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154 views

Derivation of the Hartree-Fock equations. Functional variation

I would like to ask a question about mathematical derivation of the HF equations. Some moments related to the functional variation are not clear for me. I read a book of Szabo and Ostlund "Modern ...
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1answer
604 views

Calculating the probability of finding an electron between $0.25L$ and $0.75L$ in “particle in a box problem”

If the wavefunction of the electron in a box of length L is $$\psi{(x)}=\sqrt{\frac{2}{L}}\cdot\sin{\frac{n\pi x}{L}}$$ What would be the probability of finding the electron between $0.25L$ and ...
2
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1answer
63 views

Gaussian vs ABINIT for solids

I see that Gaussian has feature to set periodic boundary conditions specifying the parameter Tv in the input file. Does it do it via the plane wave basis set? Also, the question is whether it is ...
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163 views

The ionization energy of $\ce{H2^+}$ molecule ion

I am trying to compute the ionization energy of $H_2^+$ molecule ion from the electronic energy spectrum. The question is whether one should use the purely electronic Hamiltonian or the Hamiltonian ...
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143 views

Which computational method/basis set should I use for small molecular ions?

I apologize for a perhaps newbie question. I would like to compute the potential energy surface for several small molecular ions, like $\ce{H2+}$ and $\ce{H3+}$, using quantum chemistry software. ...
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76 views

Special electron configurations in p block on periodic table?

Gold and Chromium have weird electron configurations, because electrons are differently divided : eg an electron jumps from a s-orbital to a d-orbital to have a half-filled s orbital and a fully/half ...
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Have we ever created artificial metals?

I really don't know anything about chemistry, but I was wondering about the following vague question: has any substance been created that does not contain any metallic elements, but is metallic? In ...
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CI without Hartree-Fock SCF

Usually textbooks on CI takes the SCF Hartree-Fock orbitals as a starting point. Can one use the configuration interaction method starting directly from the single-particle Slater determinants ...