Table of Contents

- 1 How the Born-Oppenheimer approximation is used?
- 2 Why do we need Born-Oppenheimer approximation?
- 3 What are the limitations of Born-Oppenheimer approximation?
- 4 What is the Schrödinger equation for the electron when the Born Oppenheimer approximation is used?
- 5 What is the Schrödinger equation for the electron when the Born-Oppenheimer approximation is used?
- 6 When Born Oppenheimer approximation breaks down?
- 7 What is the starting point in density functional theory calculations?
- 8 How are Hamiltonian operators calculated?

## How the Born-Oppenheimer approximation is used?

The Born-Oppenheimer Approximation is the assumption that the electronic motion and the nuclear motion in molecules can be separated. It leads to a molecular wave function in terms of electron positions and nuclear positions. The proton, itself, is approximately 2000 times more massive than an electron.

## Why do we need Born-Oppenheimer approximation?

The Born-Oppenheimer approximation is one of the basic concepts underlying the description of the quantum states of molecules. This approximation makes it possible to separate the motion of the nuclei and the motion of the electrons.

**Are there instances where the Born-Oppenheimer approximation breaks down for some molecules?**

In fact, the BO approximation breaks down in some cases when non-adiabatic effects are not negligible (e.g. loss or gain of energy due to changes in electronic orbits).

### What are the limitations of Born-Oppenheimer approximation?

The original BO approach had certain limitations: • They considered only stationary states, i.e., the time-independent SE. They considered only stable molecules (those having a configuration in which the forces on the nuclei vanish) and relatively small displace- ments of the nuclei from equilibrium.

### What is the Schrödinger equation for the electron when the Born Oppenheimer approximation is used?

The Schrödinger equation, which must be solved to obtain the energy levels and wavefunction of this molecule, is a partial differential eigenvalue equation in the three-dimensional coordinates of the nuclei and electrons, giving 3 × 12 + 3 × 42 = 36 nuclear + 126 electronic = 162 variables for the wave function.

**What is Hamiltonian operator in chemistry?**

The Hamiltonian operator is the sum of the kinetic energy operator and potential energy operator. The kinetic energy operator is the same for all models but the potential energy changes and is the defining parameter.

#### What is the Schrödinger equation for the electron when the Born-Oppenheimer approximation is used?

#### When Born Oppenheimer approximation breaks down?

We reiterate that when two or more potential energy surfaces approach each other, or even cross, the Born–Oppenheimer approximation breaks down, and one must fall back on the coupled equations.

**How can you calculate total energy of molecule with the help of Born Oppenheimer approximation method?**

The Born–Oppenheimer approximation assumes that the molecular wavefunction can be written in the form ψtotal=ψelectronicψvibrationψrotation and therefore that the energies due to each type of motion are additive Etotal=Eelectronic+Evibrational+Erotational.

## What is the starting point in density functional theory calculations?

Usually one starts with an initial guess for n(r), then calculates the corresponding Vs and solves the Kohn–Sham equations for the φi. From these one calculates a new density and starts again. This procedure is then repeated until convergence is reached.

## How are Hamiltonian operators calculated?

The Hamiltonian operator, H ^ ψ = E ψ , extracts eigenvalue E from eigenfunction ψ, in which ψ represents the state of a system and E its energy. The expression H ^ ψ = E ψ is Schrödinger’s time-independent equation.