It should be noted that the suggested model allowing to take into account the
effects related to the overlap of shells of valence electrons is based on the point
model of interacting molecules and can be applied only to small molecules. Such
limitation is due to the fact, that the parameters of exchange interaction β A and β B
are defined only by the ionization potentials of molecules and take into account
neither the form nor the size of interacting molecules.
3.2.4 Dipole Moment Surface of the Ethylene Dimer
Ethylene dimer in contrast to the CH 4 –N 2 complex is studied significantly wider.
The history of quantum-mechanical calculations of the interaction energy of two
ethylene molecules begins with the work of Hashimoto and Isobe in 1973 [74].
Since then a number of theoretical works [75–94] have been devoted to the ab initio
calculations of the potential energy of the dimer. The other approach to the
investigation of the potential energy surface of the C 2 H 4 –C 2 H 4 complex was
applied in Refs. [95–99] in the framework of analytical description of long-range
interactions between two ethylene molecules. This approach gives a physically
correct analytical description of the potential energy surface for interacting C 2 H 4
molecules at large intermolecular separations, R. However, in the framework of this
approach, the well depths of different configurations of the C 2 H 4 –C 2 H 4 dimer can
not be described correctly, because at these intermolecular separations the electron
shells of interacting ethylene molecules begin to overlap and the exchange interactions start to play an important role.
In contrast to the potential energy, the dipole moment of the ethylene dimer has
been investigated not sufficiently. It is known that the dipole moment of ethylene
dimer being in the most stable configuration (possessing symmetry D 2d ) equals to
zero. The theoretical calculations of the dipole moment surface of ethylene dimer
have been carried out in the work [100]. There are also experimental works devoted
to the collision-induced absorption in ethylene in the infrared region [101–104] that
contain the information on the dipole moment surface of the interacting ethylene
molecules. These collision-induced absorption spectra were used only for the
evaluation of the quadrupole moment of single ethylene molecule.
In this section, following to the work [100] the results of the analytical and
high-level ab initio calculations of the dipole moment for selected configurations of
the ethylene dimer are discussed. For the analytical calculations of the dipole
moment the monomer properties calculated in Ref. [100] are used (see Table 3.5).
A. Results of the PES and DMS calculations
For the accurate ab initio study of potential energy and dipole moment of the
C 2 H 4 –C 2 H 4 complex the methods considering the electron correlation should be
used. For this reason, the following quantum-mechanical methods were employed:
CCSD(T), CCSD(T)-F12, and MP2. All ab initio calculations in this Section were
3.2 Dipole Moment of van der Waals Complexes
39
effects related to the overlap of shells of valence electrons is based on the point
model of interacting molecules and can be applied only to small molecules. Such
limitation is due to the fact, that the parameters of exchange interaction β A and β B
are defined only by the ionization potentials of molecules and take into account
neither the form nor the size of interacting molecules.
3.2.4 Dipole Moment Surface of the Ethylene Dimer
Ethylene dimer in contrast to the CH 4 –N 2 complex is studied significantly wider.
The history of quantum-mechanical calculations of the interaction energy of two
ethylene molecules begins with the work of Hashimoto and Isobe in 1973 [74].
Since then a number of theoretical works [75–94] have been devoted to the ab initio
calculations of the potential energy of the dimer. The other approach to the
investigation of the potential energy surface of the C 2 H 4 –C 2 H 4 complex was
applied in Refs. [95–99] in the framework of analytical description of long-range
interactions between two ethylene molecules. This approach gives a physically
correct analytical description of the potential energy surface for interacting C 2 H 4
molecules at large intermolecular separations, R. However, in the framework of this
approach, the well depths of different configurations of the C 2 H 4 –C 2 H 4 dimer can
not be described correctly, because at these intermolecular separations the electron
shells of interacting ethylene molecules begin to overlap and the exchange interactions start to play an important role.
In contrast to the potential energy, the dipole moment of the ethylene dimer has
been investigated not sufficiently. It is known that the dipole moment of ethylene
dimer being in the most stable configuration (possessing symmetry D 2d ) equals to
zero. The theoretical calculations of the dipole moment surface of ethylene dimer
have been carried out in the work [100]. There are also experimental works devoted
to the collision-induced absorption in ethylene in the infrared region [101–104] that
contain the information on the dipole moment surface of the interacting ethylene
molecules. These collision-induced absorption spectra were used only for the
evaluation of the quadrupole moment of single ethylene molecule.
In this section, following to the work [100] the results of the analytical and
high-level ab initio calculations of the dipole moment for selected configurations of
the ethylene dimer are discussed. For the analytical calculations of the dipole
moment the monomer properties calculated in Ref. [100] are used (see Table 3.5).
A. Results of the PES and DMS calculations
For the accurate ab initio study of potential energy and dipole moment of the
C 2 H 4 –C 2 H 4 complex the methods considering the electron correlation should be
used. For this reason, the following quantum-mechanical methods were employed:
CCSD(T), CCSD(T)-F12, and MP2. All ab initio calculations in this Section were
3.2 Dipole Moment of van der Waals Complexes
39