# B Guessing trial wave function with variational method

1. Dec 27, 2016

### physicist 12345

Hellow

i want to ask about guessing the trial wave function at variational method of approximation

usually for example at solving harmonic oscillator or hydrogen atom we have conditions for trial wave function
but in fact i want to ask generally how could i make the guessing .. some problems give a particle of mass m moving at certain potential and want to use variational method to find gs energy now how could i guess the trial wavefunction

2. Dec 27, 2016

### A. Neumaier

By having solved a number of problems, or seen how others make the choice.

3. Dec 27, 2016

### physicist 12345

then it some thing come with experience ?

4. Dec 27, 2016

### A. Neumaier

With experience, or with trial and error. Generally one first thinks about the properties the function wanted should have (asymptotic behavior or behavior near distinguished points). Then one selects a class of functions having this property. Often there are paradigmatic exactly solvable cases that show what kind of solution is reasonable, and one can choose similar functions. Normalized wave functions typically decay exponentially. Generic variability is created by polynomial contributions. This suggests an ansatz $e^{-a|x-x_k|} p(x)$ with a polynomial $p(x)$, or (suggested by the linearity of the Schroedinger equation) linear combinations of these. If one wants to have an easy time in calculating inner products, one uses instead Gaussians times polynomials, etc..Unnormalized ones have an asymptotic form reflecting knowledge about scattering, and again one can make up trial functions with this behavior.

5. Dec 27, 2016

### marcusl

Variational methods are often not very sensitive to the choice of trial functions, so long as they're not way off.

6. Jan 9, 2017

### A. Neumaier

They must have qualitatively the correct form, and the approximation cannot be better than what the ansatz allows.