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In both molecular mechanics/dynamics calculations and quantum mechanical calculations, the literature often refers to both gas-phase calculations and calculations done in vacuo. Is there actually a difference between these, or are they the same thing? If same, why would we call something a gas-phase calculation if there's no gas there?

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    $\begingroup$ Note that in-vacuo refers to an object surrounded by a vacuum, so it isn't that nothing is there, it's that nothing is there other than the object (i.e. gas molecule) of interest. $\endgroup$ – airhuff Sep 7 '17 at 19:16
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I don't know the etymology of the phrase, but yes, "gas phase" calculations are the same as "vacuum" calculations. There is no difference.

Think of it this way: the only option other than a "gas phase" calculation is one where solvent is included, either as explicit atoms or implicitly via a mathematical model or even a solid-state/bulk-model calculation. This indicates that the solute is in the gas phase, and hypothetically not interacting with the environment in any way.

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  • $\begingroup$ What if someone wanted to model a solute in some environment with actual gas molecules? Such as to see the effects of a solute in N2? I'm not sure if that's an area of research, but am speaking hypothetically. $\endgroup$ – halcyon Sep 8 '17 at 0:53
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    $\begingroup$ You would start off by building small clusters with explicit solvent: the solute surrounded by some number of $\ce{N2}$ molecules, represented via a force field or some ab initio-derived method. But, what constitutes "gas molecules"? Small clusters are weird models used to reduce computational cost. A true "gas-solute" simulation you're thinking of would be a bulk simulation with dozens or hundreds of solvent molecules that have the density and disordered structure of a gas. $\endgroup$ – pentavalentcarbon Sep 8 '17 at 1:22
  • $\begingroup$ That makes sense, and your description is what I would think of when someone says gas phase calculation (i.e., dozens or hundreds of solvent gas molecules). It doesn't make sense to me that calculations done in vacuum are also referred to as gas phase because there is nothing other than the solute. $\endgroup$ – halcyon Sep 8 '17 at 3:13
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    $\begingroup$ "there would be low density of gas, and perhaps the solute would act as though there are no gas molecules present (hence the interchange of terms)?" - Yes, this is what I'm getting at; consider what the actual density of said gas is and the concentration of the solute, and the isolated molecule or small cluster model may be an acceptable approximation. One other thing to be aware of is that for non-dynamical simulations, such as geometry optimizations with QM methods or force fields, there is no concept of temperature, so we say that these calculations occur at 0 K. $\endgroup$ – pentavalentcarbon Sep 8 '17 at 17:21
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    $\begingroup$ If you actually wanted to study the behavior of real gases, I assume you'd need to perform non-zero temperature dynamical simulations with the appropriate solute and solvent concentrations, since the no-interaction approximation of ideal gases is no longer valid. Just to reiterate, keep in mind that this is not what is meant by "gas phase" calculation. $\endgroup$ – pentavalentcarbon Sep 8 '17 at 17:24

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