ECV_UMBRELLAS_LINE
This is part of the opes module
It is only available if you configure PLUMED with ./configure –enable-modules=opes . Furthermore, this feature is still being developed so take care when using it and report any problems on the mailing list.

Target a multiumbrella ensemble, by combining systems each with a parabolic bias potential at a different location.

Any set of collective variables \(\mathbf{s}\) can be used as ARG.

\[ \Delta u_{\mathbf{s}_i}(\mathbf{s})=\sum_j^{\text{dim}}\frac{([s]_j-[s_i]_j)^2}{2\sigma^2}\, . \]

The Gaussian umbrellas are placed along a line, from CV_MIN to CV_MAX. The umbrellas are placed at a distance SIGMA*SPACING from each other, if you need more flexibility use ECV_UMBRELLAS_FILE. The unbiased fluctuations in the basin usually are a reasonable guess for the value of SIGMA. Typically, a SPACING greater than 1 can lead to faster convergence, by reducing the total number of umbrellas. The umbrellas can be multidimensional, but the CVs dimensions should be rescaled since a single SIGMA must be used.

The keyword BARRIER can be helpful to avoid breaking your system due to a too strong initial bias. If you think the placed umbrellas will not cover the whole unbiased probability distribution you should add it explicitly to the target, with the flag ADD_P0, for more robust convergence. See also Appendix B of Ref.[75] for more details on these last two options.

The flag LOWER_HALF_ONLY modifies the ECVs so that they are set to zero when \(\mathbf{s}>\mathbf{s}_i\), as in LOWER_WALLS. This can be useful e.g. when the CV used is the ENERGY and one wants to sample a broad range of high energy values, similar to ECV_MULTITHERMAL but with a flat energy distribution as target. By pushing only from below one can avoid too extreme forces that could crash the simulation.

Examples
Click on the labels of the actions for more information on what each action computes
tested on master
cv: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between.
=1,2 ecv: ECV_UMBRELLAS_LINE
ARG
the input for this action is the scalar output from one or more other actions.
=cv
CV_MIN
compulsory keyword the minimum of the CV range to be explored
=1.2
CV_MAX
compulsory keyword the maximum of the CV range to be explored
=4.3
SIGMA
compulsory keyword sigma of the umbrella Gaussians
=0.5
SPACING
compulsory keyword ( default=1 ) the distance between umbrellas, in units of SIGMA
=1.5 opes: OPES_EXPANDED
ARG
compulsory keyword the label of the ECVs that define the expansion.
=ecv.*
PACE
compulsory keyword how often the bias is updated
=500

It is also possible to combine different ECV_UMBRELLAS_LINE to build a grid of CV values that will be sampled. For example the following code will sample a whole 2D region of cv1 and cv2.

Click on the labels of the actions for more information on what each action computes
tested on master
cv1: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between.
=1,2 ecv2: ECV_UMBRELLAS_LINE
ARG
the input for this action is the scalar output from one or more other actions.
=cv1
CV_MIN
compulsory keyword the minimum of the CV range to be explored
=1.2
CV_MAX
compulsory keyword the maximum of the CV range to be explored
=4.3
SIGMA
compulsory keyword sigma of the umbrella Gaussians
=0.5 cv2: DISTANCE
ATOMS
the pair of atom that we are calculating the distance between.
=3,4 ecv1: ECV_UMBRELLAS_LINE
ARG
the input for this action is the scalar output from one or more other actions.
=cv2
CV_MIN
compulsory keyword the minimum of the CV range to be explored
=13.8
CV_MAX
compulsory keyword the maximum of the CV range to be explored
=21.4
SIGMA
compulsory keyword sigma of the umbrella Gaussians
=4.3 opes: OPES_EXPANDED
ARG
compulsory keyword the label of the ECVs that define the expansion.
=ecv1.*,ecv2.*
PACE
compulsory keyword how often the bias is updated
=500
Glossary of keywords and components
Compulsory keywords
TEMP ( default=-1 ) temperature. If not specified tries to get it from MD engine
CV_MIN the minimum of the CV range to be explored
CV_MAX the maximum of the CV range to be explored
SIGMA sigma of the umbrella Gaussians
SPACING ( default=1 ) the distance between umbrellas, in units of SIGMA
Options
NUMERICAL_DERIVATIVES ( default=off ) calculate the derivatives for these quantities numerically
ADD_P0 ( default=off ) add the unbiased Boltzmann distribution to the target distribution, to make sure to sample it
LOWER_HALF_ONLY

( default=off ) use only the lower half of each umbrella potentials

ARG the input for this action is the scalar output from one or more other actions. The particular scalars that you will use are referenced using the label of the action. If the label appears on its own then it is assumed that the Action calculates a single scalar value. The value of this scalar is thus used as the input to this new action. If * or *.* appears the scalars calculated by all the proceeding actions in the input file are taken. Some actions have multi-component outputs and each component of the output has a specific label. For example a DISTANCE action labelled dist may have three components x, y and z. To take just the x component you should use dist.x, if you wish to take all three components then use dist.*.More information on the referencing of Actions can be found in the section of the manual on the PLUMED Getting Started. Scalar values can also be referenced using POSIX regular expressions as detailed in the section on Regular Expressions. To use this feature you you must compile PLUMED with the appropriate flag.. You can use multiple instances of this keyword i.e. ARG1, ARG2, ARG3...
BARRIER a guess of the free energy barrier to be overcome (better to stay higher than lower)