6 Wave Function-Based Correlation Methods

6.15 Using Single-Precision Arithmetic in Coupled-Cluster Calculations

Memory footprint and execution time of coupled-cluster calculations can be reduced by approximately a factor of 2 by using single-precision arithmetic.764 The errors due to using single precision are small and comparable with typical convergence thresholds.764 If loss of accuracy is undesirable, one can follow up a converged single-precision calculation with a small number of “clean-up” iterations in double precision, which recover the full accuracy of a double-precision calculation. Eventually, single-precision execution of coupled-cluster calculations will become the default, however, for now setting the single precision calculation needs to be done manually, as described below.

Single-precision capabilities are only available for the CCMAN2 suite. They work with both variants of the tensor-contraction backend (controlled by CC_BACKEND). Currently enabled features include energies, gradients, and properties calculations for both canonical and RI/CD versions. Single-precision versions of most EOM-CC methods are also available; the respective keywords are described in Section 7.10.10.

Note:  When using single-precision, the thresholds need to be adjusted accordingly, as explained below. Using too tight convergence thresholds in single precision results in a non-convergent behavior.

Note:  In rare cases, slow performance of the current code in single precision with default CC_BACKEND was noted; this issue is being investigated. Please monitor the performance and report any issues you encounter.

Webinar showing how to use the single-precision functionality in Q-Chem, including the monitoring of errors.

To deploy a single- or a mixed-precision coupled-cluster calculation, use CC_SINGLE_PREC. Its possible values are: 0 (default corresponding to double-precision calculation), 1 (single-precision calculation), and 2 (single-precision calculation followed by a couple of iterations in double precision, to recover full accuracy).

To adjust the convergence thresholds in single-precision calculation, use CC_SP_T_CONV (threshold for T and Λ amplitudes) and CC_SP_E_CONV (threshold for energies). Because too tight convergence criteria can cause non-convergent behavior, these thresholds should not be tighter than 10-6 a.u. for energies and 10-4 for amplitudes.

Calculations of intermediates, density matrices, and S2 can also be done in single precision, leading to negligible errors in energies and optimized geometries. This is controlled by the CC_SP_DM variable: 0 corresponds to calculation in double precision, 1 corresponds to calculation in single precision.

Calculation of perturbative triples corrections, (T) and (fT), can be executed in single precision using libpt; additional keywords deploying these features are USE_LIBPT and LIBPT_MIXED_PRECISION.

Note:  Calculation of numerical derivatives in single precision, such as finite difference evaluation of nuclear gradients and finite-field calculations, require using larger step sizes or field strengths, respectively.

CC_SINGLE_PREC
       Precision selection for CCSD calculation. Available in CCMAN2 only.
TYPE:
       INTEGER
DEFAULT:
       0 double-precision calculation
OPTIONS:
       1 single-precision calculation 2 single-precision calculation followed by double-precision clean-up iterations
RECOMMENDATION:
       Do not set too tight convergence thresholds when using single precision

CC_SP_T_CONV
       Amplitude convergence threshold in single precision in CCSD calculations.
TYPE:
       INTEGER
DEFAULT:
       3
OPTIONS:
       n Corresponding to 10-n convergence criterion
RECOMMENDATION:
       Set 4 to be consistent with the default threshold in double precision in a pure single-precision run. When used with clean-up version, it should be smaller than double-precision threshold not to introduce extra iterations.

CC_SP_E_CONV
       Energy convergence criterion in single precision in CCSD calculations.
TYPE:
       INTEGER
DEFAULT:
       5
OPTIONS:
       n Corresponding to 10-n convergence criterion
RECOMMENDATION:
       Set 6 to be consistent with the default threshold in double precision in a pure single-precision calculation. When used with clean-up version, it should be smaller than double-precision threshold not to introduce extra iterations.

CC_SP_DM
       Precision selection for CCSD and EOM-CCSD intermediates, density matrices, gradients, and S2
TYPE:
       INTEGER
DEFAULT:
       0 double-precision calculation
OPTIONS:
       1 single-precision calculation
RECOMMENDATION:
       NONE

CC_ERASE_DP_INTEGRALS
       Controls storage of requisite objects computed with double precision in a single-precision calculation
TYPE:
       INTEGER
DEFAULT:
       0 store
OPTIONS:
       1 do not store
RECOMMENDATION:
       Do not erase integrals if clean-up in double precision is intended.

LIBPT_MIXED_PRECISION
       Deploys single-precision evaluation of (T) and (fT) within libpt
TYPE:
       INTEGER
DEFAULT:
       0 do not use single precision
OPTIONS:
       1 use single precision
RECOMMENDATION:
       Use in combination with USE_LIBPT.

Example 6.31  A job evaluating CCSD and CCSD(T) energy using single-precision execution

$comment
Uracil+H2O cc-pvdz
CCSD(T) energy
$end

$molecule
0 1
N            .034130    -.986909    0.000000
N          -1.173397     .981920    0.000000
C          -1.218805    -.408164    0.000000
C           -.007302    1.702153    0.000000
C           1.196200    1.107045    0.000000
C           1.289085    -.345905    0.000000
O           2.310232    -.996874    0.000000
O          -2.257041   -1.026495    0.000000
H            .049329   -1.997961    0.000000
H          -2.070598    1.437050    0.000000
H           -.125651    2.776484    0.000000
H           2.111671    1.674079    0.000000
O           1.747914   -1.338382   -3.040233
H           2.180817   -1.817552   -2.333676
H           0.813180   -1.472188   -2.883392
$end

$rem
method = ccsd(t)
basis = cc-pvdz
use_libpt = true  evaluate triples with libpt code
! use SP code
  cc_single_prec=1
  eom_single_prec=1
  cc_sp_dm = 1
  cc_sp_t_conv = 4
  cc_sp_e_conv = 6
  cc_erase_dp_integrals = 1
  libpt_mixed_precision = 1
$end