Geometry Optimization

Geometry optimizations are one of the most common calculations in quantum chemistry. Let’s take a look on the geometry optimization of ethylene once again!

The ORCA output is in ethylene_geometry.out.

# import PyORCA
import pyorca as po

# parse the ORCA output file
data = po.OrcaOutput.parse_file('ethylene_geometry.out')

# let's remind ourselves, what was the ORCA input
print('The ORCA input was:')
print(data.input_text)

# geometry optimization is the only calculation in this output file
geometry_optimization = data.calculations[0]

# did the geometry optimization converge?
print('Did geometry optimization converge:', geometry_optimization.converged)
print()

# how many geometry optimization cycles did it take?
print('Number of optimization cycles:', len(geometry_optimization.cycles))
print()

# print the energies of the individual optimization cycles
for idx, cycle in enumerate(geometry_optimization):
    print(f'Cycle {idx+1}: {cycle.energy:0.2f} kJ/mol')
print()

# it is common to check the frequencies of normal modes
# any imaginary frequencies would signall that the structure is not at energetic minimum
print('Number of imaginary modes:', geometry_optimization.final_single_point.normal_modes.imaginary)
print()

# let's also take a look what is the C-C bond order
bond_orders = geometry_optimization.final_single_point.population_analysis.bond_orders
# filter the bond orders based on atom indeces
CC_bond_order = [bo for bo in bond_orders if bo.atom1==0 and bo.atom2==1][0]
print(f'The C-C Mayer bond order is {CC_bond_order.order:0.2f}')

The ORCA input was:
! B3LYP def2-TZVP def2/J OPT TightOpt FREQ

* xyz 0 1
   C       -8.44174        0.21370        0.00000
   C       -7.17489        0.62142        0.00000
   H       -8.71487       -0.72541        0.47062
   H       -9.21135        0.81722       -0.47062
   H       -6.40529        0.01790        0.47062
   H       -6.90176        1.56053       -0.47062
*

Did geometry optimization converge: True

Number of optimization cycles: 4

Cycle 1: -206273.12 kJ/mol
Cycle 2: -206273.78 kJ/mol
Cycle 3: -206274.26 kJ/mol
Cycle 4: -206274.27 kJ/mol

Number of imaginary modes: 0

The C-C Mayer bond order is 1.96

We can see that the optimization converged after 4 cycles, and that it is indeed a local minimum - there aren’t any imaginary modes.

It is possible to read many different properties from both OptimizationCycle and SinglePointCalculation.