I was reading Czycholls Solid State Theory 1 for my Quantum Dot reasearch and came across the two band Hamiltonian with hole operators. After I changed the valence band electron operators to valence band holes I got Delta-Terms after I tried to normal order the operators. What do they mean? For example the intraband Coulomb interaction for valence band electrons: $$ H_{vv}=\sum\limits_{RR'}u^{vvvv}_{RR'R'R}v^{\dagger}_{R}v^{\dagger}_{R'}v_{R'}v_{R} $$ with $v^{(\dagger)}$ being the annihilation (creation) operator of an electron in the valnce band. Now changing to the hole picture: $h^{\dagger}=v$ being the hole creation and $h=v^{\dagger}$ being the hole annihilation operator. Now $H_{vv}$ changes to: $$ H_{vv}=\sum\limits_{RR'}u^{vvvv}_{RR'R'R}(h^{\dagger}_{R'}h_{R'}h^{\dagger}_{R}h_{R}+\delta_{RR'} h^{\dagger}_{R}h_{R'}-\delta_{R'R'} h^{\dagger}_{R}h_{R}-\delta_{RR} h^{\dagger}_{R'}h_{R'}-\delta_{RR}\delta_{R'R'}-\delta_{RR'}\delta_{RR'}). $$ But in every book I look they ignore everything past the first term. Why?
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