  A new theoretical approach to study the properties in solids, which are
sensitive to a change of densities of the valence electrons in atomic cores
(hyperfine structure constants, parameters of space parity (P) and time
reversal (T) violation interaction, etc.) is proposed and implemented. It uses
the two-step concept of calculation of such properties which was implemented
earlier for the case of molecules [Progr.\ Theor.\ Chem.\ Phys. B 15, 253
(2006)]. The approach is applied to evaluate the parameter $X$ describing
electronic density gradient on the Pb nucleus that is required to interpret the
proposed experiment on PbTiO$_3$ crystal [PRA, 72, 034501 (2005)] to search for
the Schiff moment of the $^{207}$Pb nucleus because of its high sensitivity to
the corresponding P,T-violating interactions. For comparison the $X$ parameter
has also been calculated on the Pb nucleus for the $^1\Sigma^+$ state of
$^{207}$PbO molecule using the same density functionals as those utilized in
PbTiO$_3$ studies. The relativistic coupled-clusters approach with single,
double and perturbative triple cluster amplitudes, applicable to a few atom
systems and providing high accuracy for $X$, is also applied to the PbO case to
estimate the accuracy of density functional studies.
