Variational Method in Quantum Mechanics

Table of Contents

1. Introduction
2. Motivation and Importance
3. The Variational Principle
4. Statement of the Theorem
5. Constructing a Trial Wavefunction
6. Applying the Variational Method
7. Example: Ground State of the Hydrogen Atom
8. Example: Helium Atom Approximation
9. Rayleigh-Ritz Variational Method
10. Choosing Good Trial Functions
11. Variational Bounds and Limits
12. Variational Method in Quantum Field Theory
13. Applications in Quantum Chemistry
14. Limitations of the Method
15. Conclusion

1. Introduction

The variational method is a powerful approximation technique in quantum mechanics used to estimate the ground-state energy of complex systems. It is especially valuable when the Schrödinger equation cannot be solved exactly, such as for many-electron atoms and molecules.

2. Motivation and Importance

Exact solutions are rare in quantum mechanics. For most real-world systems, including atoms with more than one electron, molecules, and solids, analytical solutions are impossible. The variational method provides a way to estimate energies by optimizing trial wavefunctions, making it an essential tool in theoretical and computational physics.

3. The Variational Principle

The core of the method is the variational principle, which states:

For any normalized trial wavefunction \( |\psi_{\text{trial}}\rangle \), the expectation value of the Hamiltonian provides an upper bound to the ground-state energy \( E_0 \):

\[
E_0 \leq \langle \psi_{\text{trial}} | \hat{H} | \psi_{\text{trial}} \rangle
\]

Equality holds only if \( |\psi_{\text{trial}}\rangle \) is the true ground-state wavefunction.

4. Statement of the Theorem

Let \( \hat{H} \) be a Hermitian Hamiltonian and \( |\psi_{\text{trial}}\rangle \) a normalized state:

\[
\langle \psi_{\text{trial}} | \psi_{\text{trial}} \rangle = 1
\]

Then:

\[
E[\psi_{\text{trial}}] = \langle \psi_{\text{trial}} | \hat{H} | \psi_{\text{trial}} \rangle \geq E_0
\]

This principle holds because any non-exact wavefunction has contributions from excited states that increase the energy.

5. Constructing a Trial Wavefunction

Key considerations:

• Must satisfy the same boundary conditions and symmetries as the true wavefunction.
• Should include variational parameters to allow optimization.
• Simpler functions often yield good estimates with minimal effort.

6. Applying the Variational Method

Steps:

1. Choose a trial wavefunction \( \psi(\vec{r}; \alpha_1, \alpha_2, …) \).
2. Compute the energy functional:

\[
E[\psi] = \frac{\langle \psi | \hat{H} | \psi \rangle}{\langle \psi | \psi \rangle}
\]

3. Minimize \( E[\psi] \) with respect to the parameters \( \alpha_i \).

7. Example: Ground State of the Hydrogen Atom

Use trial wavefunction:

\[
\psi(r) = A e^{-\alpha r}
\]

where \( \alpha \) is a variational parameter. Evaluate:

\[
E(\alpha) = \frac{\int \psi^* \hat{H} \psi \, d^3r}{\int |\psi|^2 \, d^3r}
\]

and minimize with respect to \( \alpha \). The result is close to the true ground state energy \( -13.6 \, \text{eV} \).

8. Example: Helium Atom Approximation

For helium, exact solutions are not possible. Use a trial wavefunction:

\[
\psi(r_1, r_2) = e^{-\alpha r_1} e^{-\alpha r_2}
\]

Minimize the total energy with respect to \( \alpha \), accounting for electron-electron repulsion. This gives a very good estimate of the ground state energy.

9. Rayleigh-Ritz Variational Method

This method generalizes the variational principle:

• Expand the trial function in terms of basis functions:

\[
\psi = \sum_n c_n \phi_n
\]

• Construct the matrix:

\[
H_{mn} = \langle \phi_m | \hat{H} | \phi_n \rangle
\]

Solve the resulting eigenvalue problem to find optimal energies and coefficients.

10. Choosing Good Trial Functions

Tips for selecting effective trial wavefunctions:

• Include the correct asymptotic behavior.
• Incorporate physical intuition (e.g., shielding, correlation).
• Start simple and refine iteratively.
• Use known solutions as basis functions (e.g., hydrogenic orbitals).

11. Variational Bounds and Limits

• The variational estimate is always an upper bound.
• The tighter the bound, the closer the trial function is to the true wavefunction.
• Provides error estimates and confidence in approximations.

12. Variational Method in Quantum Field Theory

The variational method extends beyond non-relativistic quantum mechanics:

• Used in quantum field theory to study vacuum structure.
• Variational ansatz functions describe vacuum fluctuations and condensates.

13. Applications in Quantum Chemistry

• Used in Hartree-Fock theory and configuration interaction methods.
• Central to density functional theory (DFT).
• Helps compute molecular orbitals, binding energies, and spectra.

14. Limitations of the Method

• Only gives ground-state estimates (not excited states unless modified).
• Highly dependent on the choice of trial function.
• Optimization can become numerically intensive with many parameters.
• May converge to a local minimum instead of the global one.

15. Conclusion

The variational method is an elegant and practical approximation tool in quantum mechanics. It provides accurate estimates for ground-state energies and insights into wavefunction structures. Its wide applicability in atomic, molecular, and condensed matter physics underscores its foundational importance. With thoughtful choice of trial functions and rigorous optimization, the variational method remains indispensable in modern theoretical physics.