Counter-poise method

The beneficial effect of the counter-poise method is thus taken into account in our method without the need of additional (i.e. CP corrected) calculations. 

PaneLfb) illustrates the counter-poise method, in whidi the calculations for a single subsystem are performed within the full atomic basis set 12 the orbitals centered on it and what are called ghost orbitals centered on the partner. 

To remove the BSSE, we may consider the use of the basis set not only for Eab but also for Ea and Eb- This procedure, called the counter-poise method, was first introduced by Boys and... 

In the supermolecular method, there is a need to compensate for what is called the basis set superposition error (BSSE). The error appears because due to the incompleteness of the atomic basis set b)> Ihe individual subsystem A with the interaction switched off profits from the basis set only, while when interacting, the energy is lowered due to the total 2 U 2g basis set (the same pertains to any of the subsystems). As a result a part of the calculated interaction energy does not come from the interaction, but from the problon of the basis set used (BSSE) described above. A remedy is called the counter-poise method, in which aU quantities (including the energies of the individual subsystems) are calculated within the 2 1 U basis set. 

Fig. 13.3. (a) Basis set superposition problem (BSSE). Each of the molecules offers its own atomic orbitals to the total basis set fl = Ufl . Fig. (b) ilhjstrates the counter-poise method, in which the... 

To remove the BSSE we may consider the use of the basis set ft not only for ab but also for a and This procedure called the counter-poise method, was first introduced by Boys and Bernardi. Application of the full basis set ft when calculating a results in the wave function of A containing not only its own atomic orbitals, but also the atomic orbitals of the ( absent ) partner B, the " ost orbitals (Fig. 13.3b). As a by-product, the charge density of A exhibits broken symmetry... 

In order to avoid the basis set superposition error using the counter-poise method ... 

In the present work we summarize and continue a systematic study on van der Waals (vdW) interacting molecules using separated molecular orbitals (SMOs). Since the avoidance of basis set superposition error (BSSE) ([1] and reference therein) turned out to be one of the main problems in the study of vdW systems, we pay special attention to this question. It is known that the counter-poise (CP) method [2] is often used in order to correct the BSSE. 

It has also been shown that using localized representation by the so-called LMBPT method [41], the interaction energy in van der Waals (vdW) systems can be treated in a straightforward manner [40,42-52]. A large variety of atomic and molecular, dimer and more components systems has been studied in the framework of the LMBPT procedure. It is known that the treatment of vdW systems reveals the introduction of counter-poise (CP) calculations due to the basis set superposition error [53]. The CP calculations, however, seriously increase the computational work (see, e.g. [54-56] and [57] and references therein). The introduction of separated molecular orbitals (SMOs) allows that the basis set superposition error could be taken into account without any additional (CP) calculation. The energy terms as compared at the HF level for several systems — resulted in the SMO as well as in the CP calculations — also affirmed this conclusion [43,49]. 

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