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valtorEN
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Homework Statement
Derive the general expression of 3rd-order perturbation energy for a
non-degenerate quantum system.
Homework Equations
for nth order we have
(Ho-Eo)|n>+(H'-E1)|n-1> -E2\n-2>-En|0>=0 (given)
also,
<0|0>=1,
<1|0> = <0|1>=0,
<0|2>=<2|0>=-1/2<1|1>
<0|n>=<n|0>=-1/2*(<n-1|1>+<n-2|2>+...+<2|n-2>+<1|n-1>),
The Attempt at a Solution
for n=3 we get
(Ho-Eo)|3>+(H'-E1)|2>-E2|1>-E3|0>=0
now, multiply by <0| (this
we get <0|(H0-E0)|3> +<0|(H'-E1)|2>-<0|E2|1>-<0|E3|0>=0
by the rules above, the third term is = 0, the fourth is just =-E3 and the 2nd
is -1/2<1|1>
this gives E3=<0|(Ho-Eo)|3>-1/2*<1|(H'-E1)|1>
for <0|n>=<n|0>=-1/2(<n-1|1>+<n-2|2>+...+<2|n-2>+<1|n-1>)
for n=3
i get
E3=-1/2(<2|(H0-E0)|1>+<1|(H0-E0)|2>+<0|(H0-E0)|3>+<2|(H0-E0)|1>
+<1|(H0-E0)|2>)-1/2*(<1|(H'-E1)|1>) (phew!)
I am not sure what do do next
what is <1|2>=<2|1> = to?
i can see all the terms only have states 1 and 2 involved in them except the
3rd term <0|3> and the last term <1|1>, are these important?
does <1|1>=0?