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(May 16, 2021)

Q-Chem currently offers the possibility of accelerating RI-MP2 calculations using graphics processing units (GPUs). Currently, this is implemented for CUDA-enabled NVIDIA graphics cards only, such as (in historical order from 2008) the GeForce, Quadro, Tesla and Fermi cards. More information about CUDA-enabled cards is available at

http://www.nvidia.com/object/cuda_gpus.html

It should be noted that these GPUs have specific power and motherboard requirements.

Software requirements include the installation of the appropriate NVIDIA CUDA driver (at least version 1.0, currently 3.2) and linear algebra library, CUBLAS (at least version 1.0, currently 2.0). These can be downloaded jointly in NVIDIA’s developer website:

We have implemented a mixed-precision algorithm in order to get *better than*
single precision when users only have single-precision GPUs. This is
accomplished by noting that RI-MP2 matrices have a *large* fraction of
numerically “small” elements and a *small* fraction of numerically
“large” ones. The latter can greatly affect the accuracy of the calculation
in single-precision only calculations, but calculation involves a relatively
small number of compute cycles. So, given a threshold value $\delta $, we
perform a separation between “small” and “large” elements and accelerate
the former compute-intensive operations using the GPU (in single-precision) and
compute the latter on the CPU (using double-precision). We are thus able to
determine how much double-precision we desire by tuning the $\delta $
parameter, and tailoring the balance between computational speed and accuracy.

CUDA_RI-MP2

Enables GPU implementation of RI-MP2

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

FALSE
GPU-enabled MGEMM off
TRUE
GPU-enabled MGEMM on

RECOMMENDATION:

Necessary to set to 1 in order to run GPU-enabled RI-MP2

USECUBLAS_THRESH

Sets threshold of matrix size sent to GPU
(smaller size not worth sending to GPU).

TYPE:

INTEGER

DEFAULT:

250

OPTIONS:

n
user-defined threshold

RECOMMENDATION:

Use the default value. Anything less can
seriously hinder the GPU acceleration

USE_MGEMM

Use the mixed-precision matrix scheme (MGEMM) if you want to make calculations
in your card in single-precision (or if you have a single-precision-only GPU),
but leave some parts of the RI-MP2 calculation in double precision)

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

FALSE
MGEMM disabled
TRUE
MGEMM enabled

RECOMMENDATION:

Use when having single-precision cards

MGEMM_THRESH

Sets MGEMM threshold to determine the separation
between “large” and “small” matrix elements.
A larger threshold value will result in a value closer
to the single-precision result. Note that the desired factor
should be multiplied by 10000 to ensure an integer value.

TYPE:

INTEGER

DEFAULT:

10000
(corresponds to 1)

OPTIONS:

$n$
User-specified threshold

RECOMMENDATION:

For small molecules and basis sets up to triple-$\zeta $, the
default value suffices to not deviate too much from the
double-precision values. Care should be taken to reduce
this number for larger molecules and also larger basis-sets.

$molecule 0 1 c h1 c 1.089665 h2 c 1.089665 h1 109.47122063 h3 c 1.089665 h1 109.47122063 h2 120. h4 c 1.089665 h1 109.47122063 h2 -120. $end $rem METHOD rimp2 BASIS cc-pvdz AUX_BASIS rimp2-cc-pvdz CUDA_RIMP2 1 $end

$molecule 0 1 c h1 c 1.089665 h2 c 1.089665 h1 109.47122063 h3 c 1.089665 h1 109.47122063 h2 120. h4 c 1.089665 h1 109.47122063 h2 -120. $end $rem METHOD rimp2 BASIS cc-pvdz AUX_BASIS rimp2-cc-pvdz CUDA_RIMP2 1 USE_MGEMM 1 MGEMM_THRESH 10000 $end