  Compound-tunable embedding potential (CTEP) method is proposed. A fragment of
some chemical compound, "main cluster" in the present paper, is limited by
boundary anions such that the nearest environmental atoms are cations. The CTEP
method is based on constructing the embedding potential as linear combination
of short-range "electron-free" spherical "tunable" pseudopotentials for cations
from nearest environment of the main cluster, whereas the long-range CTEP part
consists of Coulomb potentials from optimized fractional point charges centered
on both environmental cations and anions.
  A pilot application of the CTEP method to the fersmite crystal,
CaNb$_2$O$_6$, is performed and a remarkable agreement of the electronic
density and interatomic distances within the fragment with those of the
original periodic crystal calculation is attained. Characteristics of
"atoms-in-compounds" which are of great importance for compound of $f$- and
$d$-elements (Nb in fersmite) are considered on examples of chemical shifts of
$K_{{\alpha}_{1,2}}\ $ and $K_{{\beta}_{1,2}}\ $ lines of X-ray emission
spectra in niobium. A very promising potential of this approach in studying
variety of properties of point defects containing $f$- and heavy $d$-elements
with relativistic effects, extended basis set and broken crystal symmetry
considered is discussed.
