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Van Aggelen, Helen  
2010 July 8  
A correct description of dissociating bonds is even more challenging to methods based on the density or first or second order density matrices than to wave function based techniques. Density and density matrix based techniques typically yield dissociated states with fractional charges instead of correct integer charges. Such nonphysical fractionally charged dissociated states also occur in variational second order density matrix theory[1] under the necessary but not sufficient P, Q and Gcondition for Nrepresentability. Additional Nrepresentability constraints are needed to correct them. To this end, we introduced linear constraints on the energy of atomic subspaces in the molecule. This work focuses on the implementation of such subspace constraints, more specifically on the identification of the tightest set of subspace constraints, their scaleup to bigger molecules and their relationship to sizeconsistency. These issues are discussed in relation to the potential energy surface of the F3  ion. First of all, the subspace constraints enforce sizeconsistency and a correct dissociation, but do not improve the accuracy of the method at short bond lengths, where they are not active. They only become active when one or more bonds are stretched. Furthermore, while only a small number of subspace constraints suffices to obtain the correct dissociation, the constraints are geometry dependent. They are therefore quite laborious to apply to large potential energy surfaces  
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Van Aggelen, H. (2010). Describing dissociation in variational second order density matrix theory. A â€™IX Girona Seminarâ€™. Girona: Universitat. [Consulta 7 setembre 2010]. Disponible a: http://hdl.handle.net/10256.1/1729  
http://hdl.handle.net/10256.1/1729  
eng  
Universitat de Girona. Departament de QuÃmica Universitat de Girona. Institut de QuÃmica Computacional 

IX Girona Seminar  
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Describing dissociation in variational second order density matrix theory  
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