# Homework Help: Simple question on minimising the trial wavefunction

1. Dec 4, 2014

### rwooduk

1. The problem statement, all variables and given/known data
After a calculation of the lowest energy using two variational parameters a and b it is found that: $$E_{T}(a,b) = 2a^{2} + 16b^{2}+a$$
What is the optimal (minimum) value of $$E_{T}$$

2. Relevant equations
It's just derivation.

3. The attempt at a solution
$$\frac{\delta E_{T}}{\delta a} = 4a + 1 = 0$$

therefore a= -1/4

$$\frac{\delta E_{T}}{\delta b} = 32b = 0$$

therefore b=0

when the values are put into ET I get zero?

$$E_{T}(a',b') = 2 (-\frac{1}{4})^{2} - \frac{1}{4} = 0$$

why would it be zero?

also were were told to take the second derivative to find the inflection? why would we do this? what does it tell us?

Thanks in advance for any help

2. Dec 4, 2014

### Orodruin

Staff Emeritus
It is not zero. You did a computational error:

2(1/4)^2 -1/4 = 2/16 - 1/4 = 1/8 -2/8 = -1/8

3. Dec 5, 2014

### rwooduk

hm that would explain it, many thanks for pointing this out, appreciated!

Any idea as to why the second derivative is taken and what it would tell us about the system?

4. Dec 20, 2014

### rwooduk

i'm probably being really stupid here but what would I do with something like:

ET (a,b,c) = (a+b)2 - ab + c4

which gives:

dE/da = 2a + b = 0
dE/db = 2b + a = 0

i can see it has solutions but what values should I use?