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Solvation

The Solv module in MAPLE incorporates solvent effects into calculations, transitioning from ideal gas-phase conditions to more realistic solution-phase environments. It offers two complementary approaches: an implicit solvent model based on geometry-dependent Born radii (GBSA) and an explicit solvation method that places solvent molecules around the solute from pre-built crystal box templates. Both methods can be combined to capture long-range dielectric effects alongside localized solvation phenomena such as hydrogen bonding.

GBSA Implicit Solvation

The Generalized Born with Solvent-Accessible Surface Area (GBSA) model is an implicit solvation method that represents the solvent as a continuous dielectric medium rather than discrete molecules. It accounts for two main contributions to the solvation free energy:

  • Electrostatic component (GB): Coulombic interactions between atomic partial charges are screened by the dielectric constant of the solvent. Geometry-dependent Born radii are computed for each atom to model the effective distance from the solute-solvent boundary.
  • Non-polar component (SA): The solvent-accessible surface area is used to estimate the cavitation and van der Waals contributions to the solvation free energy.

When GBSA is enabled, atomic partial charges are automatically computed using the charge equilibration (QEq) method. These charges are then used in the GB energy evaluation.

#model = uma
#solv(implicit=gbsa)
#opt

Parameters

The #solv command accepts several parameters to control the solvation setup. Parameters are passed in parentheses, separated by commas.

Parameter Default Description
implicit None Set to gbsa to enable the GBSA implicit solvent model
explicit None Solvent name for explicit solvation (e.g., water, methanol)
radius 10.0 Radius of the solvation shell around the solute, in Angstroms
clash_cutoff 1.5 Minimum distance between solvent and solute atoms; closer molecules are removed (Angstroms)
fix_dis 8.0 Distance threshold beyond which solvent molecules are frozen in place (Angstroms)

Supported Solvents

Implicit Solvent Models (GBSA)

The following solvents are available for implicit GBSA solvation, each with a calibrated dielectric constant:

  • acetone
  • acetonitrile
  • benzene
  • cs2
  • dichloromethane
  • dmf
  • dmso
  • ether
  • methanol
  • nhexan
  • thf
  • toluene
  • trichloromethane
  • water

Explicit Solvent Templates

A broad set of explicit solvent templates is available, built from pre-equilibrated crystal box structures. Some commonly used solvents include:

  • water
  • methanol
  • ethanol
  • acetone
  • acetonitrile
  • benzene
  • chloroform
  • cyclohexane
  • dichloroethane
  • diethylether
  • dimethylformamide
  • dimethylsulfoxide
  • heptane
  • hexane
  • isopropanol
  • methylenechloride
  • octane
  • pentane
  • pyridine
  • tetrahydrofuran
  • toluene
  • xylene

Over 80 additional organic solvents are available. See the MAPLE distribution for the complete list of PDB templates in the solvent library.

Explicit Solvation

Explicit solvation places individual solvent molecules around the solute using pre-built crystal box templates. The process works as follows:

  1. A solvent crystal box (PDB template) is loaded for the chosen solvent.
  2. The box is replicated to fill a sphere of the specified radius around the solute center of mass.
  3. Solvent molecules whose atoms are closer than clash_cutoff to any solute atom are removed.
  4. Solvent molecules farther than fix_dis from the solute center are frozen in place, while inner-shell solvent molecules remain free to relax.
#model = uma
#solv(explicit=water, radius=12.0, clash_cutoff=1.5, fix_dis=8.0)
#opt(method=lbfgs)

Tip

You can combine implicit and explicit solvation in the same calculation. The implicit GBSA correction is applied to the entire system (solute + explicit solvent), which can help capture long-range dielectric effects beyond the explicit solvation shell.

Usage Example

The following is a complete MAPLE input file demonstrating solvation for an acetaldehyde optimization in implicit water:

#model = uma
#device = gpu0
#solv(implicit=gbsa)
#opt(method=lbfgs, convergence=tight)

0 1
C    0.000000    0.000000    0.000000
C    1.501000    0.000000    0.000000
O    2.098000    1.060000    0.000000
H   -0.392000    1.011000    0.000000
H   -0.392000   -0.506000    0.875000
H   -0.392000   -0.506000   -0.875000
H    1.960000   -1.003000    0.000000

For explicit solvation with water, with outer molecules frozen:

#model = uma
#device = gpu0
#solv(explicit=water, radius=10.0, clash_cutoff=1.5, fix_dis=8.0)
#opt(method=lbfgs, convergence=medium)

0 1
C    0.000000    0.000000    0.000000
C    1.501000    0.000000    0.000000
O    2.098000    1.060000    0.000000
H   -0.392000    1.011000    0.000000
H   -0.392000   -0.506000    0.875000
H   -0.392000   -0.506000   -0.875000
H    1.960000   -1.003000    0.000000

And combined implicit + explicit solvation:

#model = uma
#device = gpu0
#solv(implicit=gbsa, explicit=water, radius=12.0, clash_cutoff=1.5, fix_dis=8.0)
#opt(method=lbfgs)

XYZ /path/to/solute.xyz