# Quantum chemistry Definition and 36 Discussions

Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry focused on the application of quantum mechanics to chemical systems. Understanding electronic structure and molecular dynamics using the Schrödinger equations are central topics in quantum chemistry.
Chemists rely heavily on spectroscopy through which information regarding the quantization of energy on a molecular scale can be obtained. Common methods are infra-red (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and scanning probe microscopy. Quantum chemistry studies the ground state of individual atoms and molecules, and the excited states, and transition states that occur during chemical reactions.
On the calculations, quantum chemical studies use also semi-empirical and other methods based on quantum mechanical principles, and deal with time dependent problems. Many quantum chemical studies assume the nuclei are at rest (Born–Oppenheimer approximation). Many calculations involve iterative methods that include self-consistent field methods. Major goals of quantum chemistry include increasing the accuracy of the results for small molecular systems, and increasing the size of large molecules that can be processed, which is limited by scaling considerations—the computation time increases as a power of the number of atoms.

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1. ### A What is meant when a phase is said to have "symmetry protected"?

What is meant when a phase is said to have "symmetry protected"?
2. ### B For what elements does Born–Oppenheimer approximation fail the most?

I would say that for the elements with the lowest atomic numbers, because these elements have their nuclei the lightest and so they can move more and their movement influence electrons more than in some heavier elements, whose nuclei move less. Am I right or not?
3. ### What's the chemistry behind a semiconductor?

This is a confusing subject for me. It's like only getting half of the story. Reading physics but not understanding how it works in chemistry makes it nonsense to me. Imagine we are creating a silicon-based semiconductor. They have covalent bonds between them, with each silicon atom having four...
4. ### Poor convergence using Fe HMOs for FMO in Gamess

Hello! I'm trying to model an interaction between ligands and a heme group using FMO in Gamess. I've tried to make HMOs for the FMOBND section of the input file (using an Fe-F complex), the HMOs are shown below: STO-3G 19 5 0 1 0.992624, 0.019366, 0.000000, 0.000000,-0.000001,-0.014838...
5. ### Normalize function - quantum chemistry

Normalize function f(r) = Nexp{-alpha*r} Where alpha is positive const and r is a vector I was just wondering if the fact that we have a vector value in our equation changes anything about the solution
6. ### Can I move a macro object at a very small distance (around 10 nm)?

Let's say I have a "nearly ideal" surface such as silicon wafer with around 5 nm roughness and I place something on it (with similar surface roughness) - then how can I move this object step by step at abysmal distances of just several nanometers, under 50 or so? I assume bar friction...
7. ### Can I get any macro object of very small thickness such as 10 nm?

I know graphene particles are small but how do I work with them separately? If you tell me easy-to-accomplish way for handling a single grain of graphene at home...OK. I was thinking about something like a very thin sheet of graphene, but in quantum measurements they are huge: micrometers and I...
8. ### I Why the linear combination of eigenfunctions is not a solution of the TISE

The linear combination of the eigenfunctions gives solution to the Schrodinger equation. For a system with time independent Hamiltonian the Schrodinger Equation reduces to the Time independent Schrodinger equation(TISE), so this linear combination should be a solution of the TISE. It is not...
9. ### A Interface between Biological Physics and Quantum Physics

Are there areas of studies which could be characterized as an interface of biological physics and quantum physics? Does such an interface even exist?
10. ### Calculating the Molecular Orbitals of a Molecular State

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 wondering how to find the molecular orbital configuration of each electronic state in addition to...
11. ### A How to choose the number of particles per site in Fermionic DMRG?

I am doing DMRG (in traditional formalism, not MPS) for Hubbard model H = -t ∑i ∑σci,σ ci+1,σ + U∑ini,σni,σ- In every iteration we add two sites to the system, but how do we set that how many particles are allowed in the system?
12. ### A Symmetry Considerations in Hartree-Fock-Roothaan Method

Hello, dear Quantum Chemists or Physicists. I have been extensively researching the restricted closed-shell Hartree-Fock method, and wrote a code that is working for diatomic molecules, like H2 or He2++. I, however, have a few unresolved questions that do not allow me to proceed further in...
13. ### B Spectroscopy of stars

Astronomers can determine the presence various elements in a star through spectroscopy of the incoming light from that star. The characteristic spectral lines of each element are formed by electrons transitioning between the various orbital energy levels around the nucleus of their atom. But...
14. ### B Spectroscopy of clear gases

To obtain spectroscopy on a gas like hydrogen or helium, an electric arc is passed through a container of the material, and then the emitted light is viewed through a prism which breaks it up into its component wavelengths. There are sharp lines which are formed, caused by excitation of the...
15. ### I Concepts to understand Quantum Tunneling

What concepts should be cleared to better understand quantum tunneling?.
16. ### Diatomic Hydrogen Molecule

Homework Statement Consider an ##H_2## molecule where the protons are separated by a wide distance R and both are located on the z-axis. Ignoring the spin degrees of freedom and treating the dipole-dipole interaction as a perturbation, use perturbation theory to estimate an upper limit for the...
17. ### A Several ground state calculations at once

Suppose I want to find the ground states corresponding to several Hamiltonian operators ##\left\{ \hat{H}_i \right\}##, which are similar to each other. As an example, let's take the ##\hat{H}_i##:s to be anharmonic oscillator Hamiltonians, written in nondimensional form (##\hbar = m = 1##) as...
18. ### I Why not describing the radiatation of atom spectrum by TDSE

The radiation of an atom was interpreted by time-independent schrodinger equation:electron was transformed from high energy level state to lower and emit a photon. Could we treat this process through a wavefunction ##{\psi}(t)##? Before emiting,the system's wavefunction is ##{\psi}(0)## and...
19. ### Flux-flux correlation function under Feynman's path integral

This is a chemically inspired problem, but the path is fully quantum mechanics and a bunch of integrals. How does one calculate fully quantum mechanical rate ($\kappa$) in the golden-rule approximation for two linear potential energy surfaces? Attempt: Miller (83) proposes...
20. ### Inquiring about Computer HW Specifications for Time-Intensive Calculations

Dear Everyone, We are a research group doing research in computational sciences. We are considering ordering new computers for our research. I was wondering if anyone could advise us regarding the hardware specifications needed to run heavy and time-expensive programs, specifically in...
21. ### I Do quantum linked rings (quantum knots) have energy?

How can we harness that energy? Is there any engine, heater, battery or other thing that uses it?
22. ### I Ionic bonding from a quantum mechanics point of view

Hi all, I have a question about the quantum nature of ionic bonds. I know some ideas about molecular bonding which can combine two ground state of H atom to get a bonding state and an anti-bonding state which in the case of H2 molecules it correspond to the covalent bonding (sharing of...
23. ### Coefficients of Secular Equations

Dear Everyone, I would like to ask a question regarding the evaluation of the coefficients in Linear Combination of Atomic Orbitals (LCAO). In Molecular Quantum Mechanics book authored by Peter Atkins and Ronald Friedman (Fourth Edition ISBN 0199274983), we are trying to calculate the...
24. ### How do I measure the binding energy of a molecule?

I read a paper on how a magnetic field of 4 T was used to break the van der Waals forces of an HeO molecule with a binding energy of 11.2 cm-1. I'll put the link to my original post below. Can someone tell me how this energy was measured? Your assistance is greatly appreciated...
25. ### A How to calculate energy levels of methane. (given v and J)

I have previously learned about the energy level calculation of some diatomic molecules, such as N2 and CO. Now I need to calculate the spherical top symmetric molecules like CH4. But I cannot find the force constants for it either, can anyone give me some suggestions about this?
26. ### Trying to run QM/MM simulation with calcium binding protein

Hello everyone, I'm a graduate student in a biophysics lab. I'm trying to run a QM/MM simulation with a protein that binds 3 Ca2+ ions, and as such I am having a devil of a time getting the SCF to converge. I've tried DIIS, KDIIS, damping, and a few other methods to try to get convergence with...
27. ### A What is the best Quantum Model to solve many body problems?

I know to describe Quantum Mechanical systems we can use: -Schrodinger equation -Feynman Path Integral method -Heisenberg Matrix formulation Well my question is, if you want to calculate molecular properties, and want to understand biochemistry (protein), you have a system with several...
28. ### I Triplet States and Wave Functions

Why is the triplet state space wave function ΨT1=[1σ*(r1)1σ(r2)-1σ(r1)1σ*(r2)] (ie. subtractive)? How does it relate to its antisymmetric nature? Also, why is this opposite for the spin wave function α(1)β(2)+β(1)α(2) (ie. additive)? And why is this one symmetric even though it describes the...
29. ### I Why is the KE operator negative in QM?

In the Hamilonian for an H2+, the kinetic energy of the electron (KE of nucleus ignored due to born-oppenheimer approximation) has a negative sign in front of it. I understand the signs for the potential energy operators but not for the KE apart from the strictly mathematical point of view. Can...
30. ### What Forces Cause Rotational Emissions?

I was curious about what forces can be applied to a molecule to cause a rotational emission.
31. ### GAMESS computing and quantum chemistry

Hi everyone, this isn't homework problem but something i would like to study and i am currently interested into. So how to start, this problem belongs more to quantum chemistry but i haven't found the topic in chemistry part of this site. I am learning to compute with program GAMESS but since...
32. ### Do we need quantum mechanics to explain biological stuff?

While I have to admit the question in its summarized version seems to be not very well defined (and naive, of an easy answer), so that I desire to at least make it sound less stupid. Or at least I hope. After reading all of this it might sound even worse though (no promises). I like reading...
33. ### What is the wave function for the whole system/atom ?

For example, let's say that psi(q) is the wave function of an electron(which describes/represents the electron) that is located in an atom and isolated/unentangled from the rest of the system. What is the wave function value of this psi(q) ? What is the wave function for that whole atom(with...
34. ### Quantum Quantum Mechanics and Quantum Chemistry books

Hi, I've been self studying physics for a few years and I'm looking for some new books, specifically on Quantum Mechanics or Quantum Chemistry. It doesn't matter if they are PDF files or actual books, and price doesn't really matter. I've read Feynmans Lectures books and I'm looking for the next...
35. ### Normalizing Wavefunction on a Ring

When normalizing a Wavefunction for a particle on a ring why is the normalization only done as dPhi? It's a particle on a ring, so shouldn't it be r*dPhi? This is my thinking, but I do not find other solutions doing this, just ignoring the r part. I understand that for a ring it's just a...
36. ### Feynman diagram question :-)

Dear community! I watched The Big Bang Theory and watched this episode. If you click on it you would get to a homepage - if you scroll down you would get to a section called Episode Questions -> Physics Bowl. According to the newspapers The Big Bang Theory have an employee who is scientist...