  Using scanning tunneling microscopy (STM), we experimentally and
theoretically investigate isolated platinum phthalocyanine (PtPc) molecules
adsorbed on atomically thin NaCl(100) vapor deposited on Au(111). We obtain
good agreement between theory and constant-height STM topography. We examine
why strong distortions of STM images occur as a function of distance between
molecule and STM tip. The images of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) exhibit, for
increasing distance, significant radial expansion due to electron propagation
in the vacuum. Additionally, the imaged angular dependence is substantially
distorted. The LUMO image has substantial intensity along the molecular
diagonals where PtPc has no atoms. In the electronic transport gap the image
differs drastically from HOMO and LUMO, even at energies very close to these
orbitals. As the tunneling becomes increasingly off-resonant, the eight angular
lobes of the HOMO or of the degenerate LUMOs diminish and reveal four lobes
with maxima along the molecular axes, where both, HOMO and LUMO have little or
no weight. These images are strongly influenced by low-lying PtPc orbitals that
have simple angular structures.
