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Modeling intermolecular interactions of physisorbed organic molecules using pair potential calculations
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Image of FIG. 1.
FIG. 1.

Molecular structure of CuPc and PTCDA. The labels refer to the atomic partial charges in Table I.

Image of FIG. 2.
FIG. 2.

Potential curves resulting from Eq. (3) for symmetric (a) and unsymmetric pairs (b). The vertical dotted lines indicate the corresponding sum of van der Waals radii.

Image of FIG. 3.
FIG. 3.

Scanning tunneling hydrogen microscopy image of the PTCDA/ Au(111) herringbone structure. The lattice spanned by the unit cell vectors and is indicated by the dotted lines. The two molecules (M1, M2) within the unit cell are rotated by 101° with respect to each other. The six neighbors of one PTCDA molecule are indicated by red and blue arrows.

Image of FIG. 4.
FIG. 4.

(a)–(e) Pair potential maps for five different rotational orientations of two PTCDA molecules between Θ Z = 0° and Θ Z = 101°. ΔZ, Θ X , and Θ Y are zero in all cases. Gray areas indicate repulsion between the molecules, red areas indicate attraction. Red and blue arrows in (a) and (e) denote the distance vectors between neighboring molecules as they were determined experimentally from the STHM image (Fig. 3). The insets in (a) and (e) illustrate the corresponding geometries. Panel (f) shows the minimum value of Φ extracted from the (ΔX, ΔY)-maps for each rotation angle Θ Z , plotted versus Θ Z . For details see text.

Image of FIG. 5.
FIG. 5.

Pair potential maps for the interaction of one molecule with its six nearest neighbors in the unit cell for PTCDA/Au(111). In (a), the result for the variation of the distance vector is shown. The allowed range lies only between 5.5 Å and 7.0 Å for ΔX and 8.5 Å and 10.5 Å for ΔY. The minimum is located precisely in the center of the unit cell. The central molecule and two of its symmetrically equivalent neighbors (at the left and right) are not drawn since their position is varied in this plot. In (b), the azimuthal orientations of both molecules were varied. Angles are measured between the long molecular axis and the long vector of the unit cell. The minimum is found for angles of 40° and 140° for the corner- and center-molecule, respectively.

Image of FIG. 6.
FIG. 6.

ΔXZ pair potential map for PTCDA molecules arranged in π-stacking direction (ΔY = 0). In the right, a cut in ΔZ direction through the minimum at ΔX = 2.8 Å and ΔY = 0 is shown. The minimum corresponds nicely to the molecular stacking geometry of a bulk crystal shown in the inset.

Image of FIG. 7.
FIG. 7.

(a) Structural phase diagram of CuPc/Au(111), taken from Ref. 10. (b) Calculated pair potential (black) and its gradient (red) plotted versus the radial displacement along the direction (distance of the two molecules) and versus coverage (nonlinear scale at the top).

Image of FIG. 8.
FIG. 8.

(a) Pair potential map of equally oriented CuPc molecules. The lattice vectors of the LT-phase structure ( Å, Å) lie close to the minima of the potential map, if the superimposed experimental unit cell and lattice of the reconstructed Au(111) surface are rotated by 26° relative to the molecular axis. (b) Vicinity of the minima of the potential map. (c) Corresponding real space model.

Image of FIG. 9.
FIG. 9.

(a) Illustration of the geometry of the two CuPc LUMO states which are degenerate in the gas phase. Their different registry with the substrate causes differences in the charge transfer with the surface and, therefore, lifts the degeneracy. (b) The favorable adsorption geometry (right) obtained from pair potential calculations agrees well with the experimental data. The empty-states STM image (left) is reproduced from Ref. 42. Dotted lines represent the molecular orientation, solid lines the direction of rows of copper atoms. (c) Pair potential map for the interaction of two equally oriented CuPc molecules including the modeled charge redistribution based on the donation/back-donation effect. (d) Pair potential map for the interaction between a third CuPc molecule that is attaching to a cluster of two molecules which are oriented according to the best configuration.


Generic image for table
Table I.

Atomic partial charges of PTCDA and CuPc derived from NBO population analysis.27 The indices refer to atomic labels in Fig. 1.

Generic image for table
Table II.

Coefficients of the van der Waals interaction and the Pauli repulsion as well as van der Waals radii used for the pair potential calculations according to Eq. (3). b n are taken from Ref. 30, c n mostly from Refs. 33 and 34, and from Ref. 35.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Modeling intermolecular interactions of physisorbed organic molecules using pair potential calculations