The density threshold truncation method is an alternative to the atom-based truncation method described in section 62.4.1. Its primary advantage is that it requires just one parameter to be adjusted for all types of molecular system and functions that are important in the long range are automatically kept. At its low truncation limit it produces the full basis embedding answer or at high truncation its limit is a result constructed from just the functions on the active embedding region. Application of this method involves slightly different input options:

`AOTRUNC`The

`AOTRUNC`option invokes the use of the density threshold method, and must always be be given when this method is used.`DENKEEP=value`(default 0.0)The

`DENKEEP`option is the minimum gross Mulliken population that each basis function is compared against. Environment functions with a greater gross populations than this value are grouped (according to angular momentum partners i.e px, with py and pz) and are kept. Environment functions lower than DENKEEP are grouped and discarded from the post embedding calculation. Setting`DENKEEP=0.0`is equivalent to the case in which the basis set is not truncated as all functions have a gross population greater than 0. For accurate type-in-type energies,`DENKEEP`should typically be no more than 0.001, although convergence with respect to this parameter slightly varies according to the system and degree of delocalisation.For example:

`{ks;orbital,2100.2}``{ibba,bonds=1;orbital,2100.2;save,2300.2}``{embed,proj,AOTRUNC,DENKEEP=0.001;orbital,2300.2,local}``{hf}``{ccsd(t)}``STOREAO`Enables the retention of selected functions along a geometry scan. This can be useful for basis set consistency for a dissociation reaction, which naturally would have fewer functions retained for a given threshold as the active and environment are separated. It is recommended that the calculations always go from associated to dissociated in order to benefit from this routine.

When using this method, the following should be taken into consideration:

(1) The accuracy of this truncation method is sensitive to the size of the basis set. For this reason, it is recommended to use at least a triple-zeta basis set.

(2) It is recommended to use Knizia's IBO localization method, which reduces the orbital tails.

(3) For accurate HF-in-HF and DFT-in-DFT results, it is recommended to use a value for`DENKEEP` of less than 0.0001. This is especially important when embedding across covalent bonds.

(4) The geometry must be in Cartesian coordinates.

(1) The accuracy of this truncation method is sensitive to the size of the basis set. For this reason, it is recommended to use at least a triple-zeta basis set.

(2) It is recommended to use Knizia's IBO localization method, which reduces the orbital tails.

(3) For accurate HF-in-HF and DFT-in-DFT results, it is recommended to use a value for

(4) The geometry must be in Cartesian coordinates.

All publications resulting from the use of this method must acknowledge the following:

S. J. Bennie, M. Stella, T. F. Miller III and F. R. Manby, J. Chem. Phys., 143, 024105 (2015).

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molpro@molpro.net 2018-04-21