Shift up a bond in ChemFig

Question

In ChemFig, the "bond join=true" option does not work with "cram" bonds (see A, it is zoomed at join).

It is possible to increase the length of the bond with the "shorten" option, but the joint still has a difference in height (see B)

Is there any option/command in tikz that can be used in the bond options to shift up a bond at a height "H" (see B)?

\documentclass[12pt,border=2mm]{standalone}
\usepackage{chemfig}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\begin{document}
\setchemfig{bond offset =1pt,bond join=true,atom sep =2em,cram width =3pt,cram dash width =0.4pt,cram dash sep =0.4pt}
    \chemfig{<[:300]-[0,,,,line width = 3pt]>[:60]}
\end{document}

Answer 1

I think, shifting the path is not that simple. It seems that the path drawn by chemfig is not a to-path, so replacement is not feasable using the to path key for example.

But what about these two ideas:

1. Use line cap=round.
2. Use Triangle Cap as arrow tip (via arrows.meta).

Another, probably somewhat too complicated solution could be to use a decoration to draw a shifted path.

\documentclass[border=10pt]{standalone}
\usepackage{chemfig}
\usetikzlibrary{arrows.meta}
\tikzset{
    sharp caps/.style={
        {Triangle Cap[length=0.8pt]}-{Triangle Cap[length=0.8pt]}, 
        shorten >=-1.9pt, 
        shorten <=-1.9pt
    }
}
\begin{document}
\setchemfig{
    bond offset=1pt,
    bond join=true,
    atom sep=2em,
    cram width=3pt,
    cram dash width=0.4pt,
    cram dash sep=0.4pt
}
    \chemfig{<[:300]-[0,,,,line width=3pt, line cap=round]>[:60]}
    \chemfig{<[:300]-[0,,,,line width=3pt, sharp caps]>[:60]}
\end{document}

Zoomed in:

Well, almost ...

Answer 2

With a little geometry we can fill the gap ...(complicated)

\documentclass[12pt,border=2mm]{standalone}
\usepackage{chemfig}
\begin{document}
    \setchemfig{atom sep=2em,cram width=3pt} % 2em=24pt
    \chemfig{<[@{A1,1}:300]-[0,,,,line width=3pt]>[@{A2,0}:60]}
    \chemmove{
        \pgfmathsetmacro{\tang}{\fpeval{atand(1.5/24)}} % \tang is the half angle of the sides of the cram bond
        \path (A1) ++(-90:1.5pt) coordinate (B1);
        \path (A1) ++(210:1.7pt) coordinate (C1);
        \path (C1) ++(300-\tang:\fpeval{0.65/cosd(300-\tang-270)}pt) coordinate (D1);
        \fill[red] (A1) -- (C1) -- (D1) -- ([xshift=1pt]B1) -- cycle; % xshift eliminates the artefact
        \path (A2) ++(-90:1.5pt) coordinate (B2);
        \path (A2) ++(-30:1.7pt) coordinate (C2);
        \path (C2) ++(-120+\tang:\fpeval{0.65/cosd(300-\tang-270)}pt) coordinate (D2);
        \fill[red] (A2) -- (C2) -- (D2) -- ([xshift=-1pt]B2) -- cycle;
    }
\end{document}

Edit

Filling the gap with a pic...

cramgap={cram width/pt}{length of the cram bond/pt}{bonds angle}

\documentclass[12pt,border=2mm]{standalone}
\usepackage{tikz}
\tikzset{pics/cramgap/.style n args={3}{code={%
    \pgfmathsetmacro{\tang}{\fpeval{atand((#1/2)/#2)}}
    \path (0,0) coordinate (cgO);
    \path (cgO) ++(-90:\fpeval{(0.4+#1)/2}pt) coordinate (cgA);
    \path (cgO) ++(\fpeval{90+#3}:\fpeval{(0.4+#1)/2}pt) coordinate (cgB);
    \path (cgB) ++(\fpeval{180+#3-\tang}:\fpeval{(0.2+#1/2)*(1-cosd(180-#3))/cosd(-90+#3-\tang)}pt) coordinate (cgX);
    \fill[red] (cgO) -- (cgB) -- (cgX) -- ([xshift=\fpeval{#3>0?1:-1}pt]cgA) -- cycle;
}}}
\usepackage{chemfig}
\begin{document}
    \setchemfig{atom sep=24pt,cram width=2.6pt} % 2em=24pt
    \chemfig{<[@{A1,1}:310]-[:0,,,,line width=3pt]>[@{A2,0}:40]} % line width=0.4pt + cram width
    \chemmove{\path (A1) pic{cramgap={2.6}{24}{130}};
        \path (A2) pic{cramgap={2.6}{24}{-140}};}
\chemfig{&lt;[@{A1,1}:330]-[:20,,,,line width=3pt]&gt;[@{A2,0}:60]}
\chemmove{\path (A1) pic[rotate=20]{cramgap={2.6}{24}{130}};
    \path (A2) pic[rotate=20]{cramgap={2.6}{24}{-140}};}

\end{document}

Answer 3

I created a way to shift up the link, but many may find it a bad solution.

One problem was not knowing the starting and ending points of the bond. Then I marked these points with "@{x}", a ChemFig feature to mark positions in molecules.

I made the bond invisible using [,,,,draw=none]

Then, using \chemmove, I shifted up the start and end points using "\node", and created a straight line to replace the bond using "\draw".

Well, technically this isn't a "chemfig bond" (-[,<length.,,,,,,]), but it uses a lot of what ChemFig already uses to build molecules and is easy to tweak the parameters.

\documentclass[12pt,border=2mm]{standalone}
\usepackage{chemfig}
\begin{document}
\setchemfig{bond offset =0.5pt,bond join=true,atom sep =2em,cram width =3pt,cram dash width =0.2pt,cram dash sep =0.4pt}
\schemestart
    \chemfig{<[:300]@{a}-[0,,,,draw=none]@{b}>[:60]}
\chemmove{
    \node[at=(a),yshift=.57pt](a){};
    \node[at=(b),yshift=.57pt](b){};
    \draw[-,line width=3pt,shorten <=-1.2pt,shorten >=-1.2pt](a)--(b);}
\schemestop
\end{document}