It has been pointed out that the exponential decrease with pressure of the photoluminescence intensity around 3GPa observed in ZnSe/ZnCdSe superlattices and quantum wells can not be explained by the direct-gap indirect-gap transformation because the direct-gap indirect-gap transformation pressures are estimated as 13.7GPa for ZnSe and 7GPa for ZnSe/Zn

1-x

Cd

x

Se(

x

=0.26) superlattices and quantum wells. But till now

no detailed calculations for the transformation pressures have been reported for the ZnSe-based heterostructures. In order to understand better the properties of ZnSe and ZnSe-based heterostructures under pressure

we studied the hydrostatic pressure dependence of the Γ

X

and

L

energy band minima for bulk ZnSe and the ZnSe epilayer of ZnSe/GaAs heterostructures using the empirical pseudopotential method. The pseudopotential form factors at various pressures are obtained by scaling the atmospheric pseudopotential form factors in terms of the electron effective mass or the energy-gap and the results show that the method of the energy-gap scaling

which is proposed in this paper for the first time

is better than the electron-effective-mass scaling. The transformation pressures of bulk ZnSe and the ZnSe/GaAs epilayer are calculated as 13.5GPa and 9.5GPa with the energy-gap scaling. By fitting the curves of the energy band minima versus pressure

we also obtained the pressure coefficients of the Γ

X

and

L

energy band minima of bulk ZnSe. The corresponding pressure coefficients are 0.211 3eV/GPa

0.071 4eV/GPa and 0.153 5eV/GPa respectively. It can be seen that the pressure coefficient of the X minima of ZnSe is positive

which is different from that of Si

Ge and GaAs semiconductor materials. On the other hand

the pressure coefficient of the X minima of ZnSe is smaller than that of the Γ minima

so the transformation from direct-gap semiconductor to indirect-gap semiconductor of bulk ZnSe can still occur but needs much greater pressure. It is also shown that the pressure coefficients of the ZnSe epilayer in a ZnSe/GaAs heterostructure are obviously affected by the build-in strain caused by lattice-mismatch between the epilayer and the substrate. This is the reason why the transformation pressure is larger for ZnSe-based heterostructure materials.