Q-Chem 4.4 User’s Manual

# 9.1 Equilibrium Geometries and Transition-State Structures

## 9.1.1 Overview

Molecular potential energy surfaces rely on the Born-Oppenheimer separation of nuclear and electronic motion. Minima on such energy surfaces correspond to the classical picture of equilibrium geometries, and transition state structures correspond to first-order saddle points. Both equilibrium and transition-state structures are stationary points for which the energy gradient vanishes. Characterization of such critical points requires consideration of the eigenvalues of the Hessian (second derivative matrix): minimum-energy, equilibrium geometries possess Hessians whose eigenvalues are all positive, whereas transition-state structures are defined by a Hessian with precisely one negative eigenvalue. (The latter is therefore a local maximum along the reaction path between minimum-energy reactant and product structures, but a minimum in all directions perpendicular to this reaction path.

The quality of a geometry optimization algorithm is of major importance; even the fastest integral code in the world will be useless if combined with an inefficient optimization algorithm that requires excessive numbers of steps to converge. Q-Chem incorporates a geometry optimization package (Optimize—see Appendix A) developed by the late Jon Baker over more than ten years.

The key to optimizing a molecular geometry successfully is to proceed from the starting geometry to the final geometry in as few steps as possible. Four factors influence the path and number of steps:

• starting geometry

• optimization algorithm

• quality of the Hessian (and gradient)

• coordinate system

Q-Chem controls the last three of these, but the starting geometry is solely determined by the user, and the closer it is to the converged geometry, the fewer optimization steps will be required. Decisions regarding the optimization algorithm and the coordinate system are generally made by the Optimize package (i.e., internally, within Q-Chem) to maximize the rate of convergence. Although users may override these choices in many cases, this is not generally recommended.

 Level of Theory Analytical Maximum Angular Analytical Maximum Angular (Algorithm) Gradients Momentum Type Hessian Momentum Type DFT ✓ ✓ HF ✓ ✓ ROHF ✓ ✗ MP2 ✓ ✗ (V)OD ✓ ✗ (V)QCCD ✓ ✗ CIS (except RO) ✓ ✓ CFMM ✓ ✗
Table 9.1: Gradients and Hessians currently available for geometry optimizations with maximum angular momentum types for analytical derivative calculations (for higher angular momentum, derivatives are computed numerically). Analytical Hessians are not yet available for meta-GGA functionals such as BMK and the M05 and M06 series.

Another consideration when trying to minimize the optimization time concerns the quality of the gradient and Hessian. A higher-quality Hessian (i.e., analytical versus approximate) will in many cases lead to faster convergence, in the sense of requiring fewer optimization steps. However, the construction of an analytical Hessian requires significant computational effort and may outweigh the advantage of fewer optimization cycles. Currently available analytical gradients and Hessians are summarized in Table 9.1.

Features of Q-Chem’s geometry and transition-state optimization capabilities include:

• Cartesian, Z-matrix or internal coordinate systems

• Eigenvector Following (EF) or GDIIS algorithms

• Constrained optimizations

• Equilibrium structure searches

• Transition structure searches

• Initial Hessian and Hessian update options

• Reaction pathways using intrinsic reaction coordinates (IRC)

• Optimization of minimum-energy crossing points (MECPs) along conical seams

## 9.1.2 Job Control

Obviously a level of theory, basis set, and starting molecular geometry must be specified to begin a geometry optimization or transition-structure search. These aspects are described elsewhere in this manual, and this section describes job-control variables specific to optimizations.

JOBTYPE
 Specifies the calculation.

TYPE:
 STRING

DEFAULT:
 Default is single-point, which should be changed to one of the following options.

OPTIONS:
 OPT Equilibrium structure optimization. TS Transition structure optimization. RPATH Intrinsic reaction path following.

RECOMMENDATION:
 Application-dependent.

GEOM_OPT_HESSIAN
 Determines the initial Hessian status.

TYPE:
 STRING

DEFAULT:
 DIAGONAL

OPTIONS:
 DIAGONAL Set up diagonal Hessian. READ Have exact or initial Hessian. Use as is if Cartesian, or transform if internals.

RECOMMENDATION:
 An accurate initial Hessian will improve the performance of the optimizer, but is expensive to compute.

GEOM_OPT_COORDS
 Controls the type of optimization coordinates.

TYPE:
 INTEGER

DEFAULT:
 1

OPTIONS:
 0 Optimize in Cartesian coordinates. 1 Generate and optimize in internal coordinates, if this fails abort. 1 Generate and optimize in internal coordinates, if this fails at any stage of the optimization, switch to Cartesian and continue. 2 Optimize in -matrix coordinates, if this fails abort. 2 Optimize in -matrix coordinates, if this fails during any stage of the optimization switch to Cartesians and continue.

RECOMMENDATION:
 Use the default; delocalized internals are more efficient.

 Convergence on maximum gradient component.

TYPE:
 INTEGER

DEFAULT:
 300 tolerance on maximum gradient component.

OPTIONS:
 Integer value (tolerance = ).

RECOMMENDATION:
 Use the default. To converge GEOM_OPT_TOL_GRADIENT and one of GEOM_OPT_TOL_DISPLACEMENT and GEOM_OPT_TOL_ENERGY must be satisfied.

GEOM_OPT_TOL_DISPLACEMENT
 Convergence on maximum atomic displacement.

TYPE:
 INTEGER

DEFAULT:
 1200 tolerance on maximum atomic displacement.

OPTIONS:
 Integer value (tolerance = ).

RECOMMENDATION:
 Use the default. To converge GEOM_OPT_TOL_GRADIENT and one of GEOM_OPT_TOL_DISPLACEMENT and GEOM_OPT_TOL_ENERGY must be satisfied.

GEOM_OPT_TOL_ENERGY
 Convergence on energy change of successive optimization cycles.

TYPE:
 INTEGER

DEFAULT:
 100 tolerance on maximum (absolute) energy change.

OPTIONS:
 Integer value (tolerance = value ).

RECOMMENDATION:
 Use the default. To converge GEOM_OPT_TOL_GRADIENT and one of GEOM_OPT_TOL_DISPLACEMENT and GEOM_OPT_TOL_ENERGY must be satisfied.

GEOM_OPT_MAX_CYCLES
 Maximum number of optimization cycles.

TYPE:
 INTEGER

DEFAULT:
 50

OPTIONS:
 User defined positive integer.

RECOMMENDATION:
 The default should be sufficient for most cases. Increase if the initial guess geometry is poor, or for systems with shallow potential wells.

GEOM_OPT_PRINT
 Controls the amount of Optimize print output.

TYPE:
 INTEGER

DEFAULT:
 3 Error messages, summary, warning, standard information and gradient print out.

OPTIONS:
 0 Error messages only. 1 Level 0 plus summary and warning print out. 2 Level 1 plus standard information. 3 Level 2 plus gradient print out. 4 Level 3 plus Hessian print out. 5 Level 4 plus iterative print out. 6 Level 5 plus internal generation print out. 7 Debug print out.

RECOMMENDATION:
 Use the default.

GEOM_OPT_SYMFLAG
 Controls the use of symmetry in Optimize.

TYPE:
 INTEGER

DEFAULT:
 1

OPTIONS:
 1 Make use of point group symmetry. 0 Do not make use of point group symmetry.

RECOMMENDATION:
 Use the default.

GEOM_OPT_MODE
 Determines Hessian mode followed during a transition state search.

TYPE:
 INTEGER

DEFAULT:
 0

OPTIONS:
 0 Mode following off. Maximize along mode .

RECOMMENDATION:
 Use the default, for geometry optimizations.

GEOM_OPT_MAX_DIIS
 Controls maximum size of subspace for GDIIS.

TYPE:
 INTEGER

DEFAULT:
 0

OPTIONS:
 0 Do not use GDIIS. -1 Default size = min(NDEG, NATOMS, 4) NDEG = number of molecular degrees of freedom. Size specified by user.

RECOMMENDATION:
 Use the default or do not set too large.

GEOM_OPT_DMAX
 Maximum allowed step size. Value supplied is multiplied by 10.

TYPE:
 INTEGER

DEFAULT:
 300 = 0.3

OPTIONS:
 User-defined cutoff.

RECOMMENDATION:
 Use the default.

GEOM_OPT_UPDATE
 Controls the Hessian update algorithm.

TYPE:
 INTEGER

DEFAULT:
 -1

OPTIONS:
 -1 Use the default update algorithm. 0 Do not update the Hessian (not recommended). 1 Murtagh-Sargent update. 2 Powell update. 3 Powell/Murtagh-Sargent update (TS default). 4 BFGS update (OPT default). 5 BFGS with safeguards to ensure retention of positive definiteness (GDISS default).

RECOMMENDATION:
 Use the default.

GEOM_OPT_LINEAR_ANGLE
 Threshold for near linear bond angles (degrees).

TYPE:
 INTEGER

DEFAULT:
 165 degrees.

OPTIONS:
 User-defined level.

RECOMMENDATION:
 Use the default.

FDIFF_STEPSIZE
 Displacement used for calculating derivatives by finite difference.

TYPE:
 INTEGER

DEFAULT:
 100 Corresponding to 0.001 . For calculating second derivatives.

OPTIONS:
 Use a step size of .

RECOMMENDATION:
 Use the default except in cases where the potential surface is very flat, in which case a larger value should be used. See FDIFF_STEPSIZE_QFF for third and fourth derivatives.

Example 9.191  As outlined, the rate of convergence of the iterative optimization process is dependent on a number of factors, one of which is the use of an initial analytic Hessian. This is easily achieved by instructing Q-Chem to calculate an analytic Hessian and proceed then to determine the required critical point

$molecule 0 1 O H 1 oh H 1 oh 2 hoh oh = 1.1 hoh = 104$end

$rem JOBTYPE freq Calculate an analytic Hessian METHOD hf BASIS 6-31g(d)$end

$comment Now proceed with the Optimization making sure to read in the analytic Hessian (use other available information too).$end

@@@

$molecule read$end

$rem JOBTYPE opt METHOD hf BASIS 6-31g(d) SCF_GUESS read GEOM_OPT_HESSIAN read Have the initial Hessian$end