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ChemOffice.Com - CambridgeSoft

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.<br />

The Molecular Dynamics dialog box appears<br />

with the default values.<br />

5. Enter the appropriate values.<br />

6. Click Run.<br />

Dynamics Settings<br />

Use the Dynamics tab to enter parameter values for<br />

the parameters that define the molecular dynamics<br />

calculations:<br />

• Step Interval—determines the time between<br />

molecular dynamics steps. The step interval<br />

must be less than ~5% of the vibration period<br />

for the highest frequency normal mode, (10 fs<br />

for a 3336 cm– 1 H–X stretching vibration).<br />

Normally a step interval of 1 or 2 fs yields<br />

reasonable results. Larger step intervals may<br />

cause the integration method to break down,<br />

because higher order moments of the position<br />

are neglected in the Beeman algorithm.<br />

• Frame Interval—determines the interval at<br />

which frames and statistics are collected. A<br />

frame interval of 10 or 20 fs gives a fairly<br />

smooth sequence of frames, and a frame<br />

interval of 100 fs or more can be used to obtain<br />

samples of conformational space over a longer<br />

computation.<br />

• Terminate After—causes the molecular<br />

dynamics run to stop after the specified<br />

number of steps. The total time of the run is<br />

the Step Interval times the number of steps.<br />

• Heating/Cooling Rate—dictates whether<br />

temperature adjustments are made. If the<br />

Heating/Cooling Rate check box is checked,<br />

the Heating/Cooling Rate slider determines<br />

the rate at which energy is added to or removed<br />

from the model when it is far from the target<br />

temperature.<br />

A heating/cooling rate of approximately 1.0<br />

kcal/atom/picosecond results in small<br />

corrections which minimally disturb the<br />

trajectory. A much higher rate quickly heats up<br />

the model, but an equilibration or stabilization<br />

period is required to yield statistically<br />

meaningful results.<br />

To compute an isoenthalpic trajectory<br />

(constant total energy), deselect<br />

Heating/Cooling Rate.<br />

• Target Temperature—the final temperature<br />

to which the calculation will run. Energy is<br />

added to or removed from the model when the<br />

computed temperature varies more than 3%<br />

from the target temperature.<br />

The computed temperature used for this<br />

purpose is an exponentially weighted average<br />

temperature with a memory half-life of about<br />

20 steps.<br />

Job Type Settings<br />

Use the Job Type tab to set options for the<br />

computation.<br />

<strong>ChemOffice</strong> 2005/Chem3D MM2 and MM3 <strong>Com</strong>putations • 155<br />

Molecular Dynamics

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