# Defining the Turbomole named BP86 functional in Gaussian [closed]

According to Turbomole, the BP86 density functional has the following definition:

$$\mathrm{Exchange = LDA + B88\\ Correlation = LDA(VWN(V)) + P86}$$

What is the equivalent definition in Gaussian using IOp keywords?

From the Gaussian 09 manual:

User-Defined Models. Gaussian 09 can use any model of the general form: P2EXHF + P1(P4EXSlater + P3ΔExnon-local) + P6EClocal + P5ΔECnon-local

The only available local exchange method is Slater (S), which should be used when only local exchange is desired. Any combinable non-local exchange functional and combinable correlation functional may be used (as listed previously).

The values of the six parameters are specified with various non-standard options to the program:

• IOp(3/76=mmmmmnnnnn) sets P1 to mmmmm/10000 and P2 to nnnnn/10000. P1 is usually set to either 1.0 or 0.0, depending on whether an exchange functional is desired or not, and any scaling is accomplished using P3 and P4.

• IOp(3/77=mmmmmnnnnn) sets P3 to mmmmm/10000 and P4 to nnnnn/10000.

• IOp(3/78=mmmmmnnnnn) sets P5 to mmmmm/10000 and P6 to nnnnn/10000.

For example, IOp(3/76=1000005000) sets P1 to 1.0 and P2 to 0.5. Note that all values must be expressed using five digits, adding any necessary leading zeros.

Here is a route section specifying the functional corresponding to the B3LYP keyword:

#P BLYP IOp(3/76=1000002000) IOp(3/77=0720008000) IOp(3/78=0810010000)

The output file displays the values that are in use:

IExCor=  402 DFT=T Ex=B+HF Corr=LYP ExCW=0 ScaHFX=  0.200000


where the value of ScaHFX is P2, and the sequence of values given for ScaDFX are P4, P3, P6 and P5.

For BP86 specified normally, this looks like:

IExCor=  404 DFT=T Ex=B Corr=P86 ExCW=0 ScaHFX=  0.000000

Those 3 IOps don't completely define the functional, however, IOp(3/74) is also needed. See the IExCor field, which corresponds to Becke 1988 exchange (400) combined with Perdew 81 + Perdew 86 correlation (4).