diff --git a/src/mol-model/structure/structure/structure.ts b/src/mol-model/structure/structure/structure.ts
index 348e8a7d9f650ba6bdd88b352fad774087d0d99f..05c5257a3866c809e73e3e7743bfd13045d16068 100644
--- a/src/mol-model/structure/structure/structure.ts
+++ b/src/mol-model/structure/structure/structure.ts
@@ -25,17 +25,28 @@ import { computeCarbohydrates } from './carbohydrates/compute';
 class Structure {
     readonly unitMap: IntMap<Unit>;
     readonly units: ReadonlyArray<Unit>;
-    /** Count of all elements in the structure, i.e. the sum of the elements in the units */
-    readonly elementCount: number;
 
-    private _hashCode = 0;
+    private _props: {
+        lookup3d?: StructureLookup3D,
+        links?: InterUnitBonds,
+        crossLinkRestraints?: CrossLinkRestraints,
+        unitSymmetryGroups?: ReadonlyArray<Unit.SymmetryGroup>,
+        carbohydrates?: Carbohydrates,
+        hashCode: number,
+        elementCount: number
+    } = { hashCode: -1, elementCount: 0 };
 
     subsetBuilder(isSorted: boolean) {
         return new StructureSubsetBuilder(this, isSorted);
     }
 
+    /** Count of all elements in the structure, i.e. the sum of the elements in the units */
+    get elementCount() {
+        return this._props.elementCount;
+    }
+
     get hashCode() {
-        if (this._hashCode !== 0) return this._hashCode;
+        if (this._props.hashCode !== -1) return this._props.hashCode;
         return this.computeHash();
     }
 
@@ -48,7 +59,8 @@ class Structure {
         }
         hash = (31 * hash + this.elementCount) | 0;
         hash = hash1(hash);
-        this._hashCode = hash;
+        if (hash === -1) hash = 0;
+        this._props.hashCode = hash;
         return hash;
     }
 
@@ -60,39 +72,34 @@ class Structure {
         return this.lookup3d.boundary;
     }
 
-    private _lookup3d?: StructureLookup3D = void 0;
     get lookup3d() {
-        if (this._lookup3d) return this._lookup3d;
-        this._lookup3d = new StructureLookup3D(this);
-        return this._lookup3d;
+        if (this._props.lookup3d) return this._props.lookup3d;
+        this._props.lookup3d = new StructureLookup3D(this);
+        return this._props.lookup3d;
     }
 
-    private _links?: InterUnitBonds = void 0;
     get links() {
-        if (this._links) return this._links;
-        this._links = computeInterUnitBonds(this);
-        return this._links;
+        if (this._props.links) return this._props.links;
+        this._props.links = computeInterUnitBonds(this);
+        return this._props.links;
     }
 
-    private _crossLinkRestraints?: CrossLinkRestraints = void 0;
     get crossLinkRestraints() {
-        if (this._crossLinkRestraints) return this._crossLinkRestraints;
-        this._crossLinkRestraints = extractCrossLinkRestraints(this);
-        return this._crossLinkRestraints;
+        if (this._props.crossLinkRestraints) return this._props.crossLinkRestraints;
+        this._props.crossLinkRestraints = extractCrossLinkRestraints(this);
+        return this._props.crossLinkRestraints;
     }
 
-    private _unitSymmetryGroups?: ReadonlyArray<Unit.SymmetryGroup> = void 0;
     get unitSymmetryGroups(): ReadonlyArray<Unit.SymmetryGroup> {
-        if (this._unitSymmetryGroups) return this._unitSymmetryGroups;
-        this._unitSymmetryGroups = StructureSymmetry.computeTransformGroups(this);
-        return this._unitSymmetryGroups;
+        if (this._props.unitSymmetryGroups) return this._props.unitSymmetryGroups;
+        this._props.unitSymmetryGroups = StructureSymmetry.computeTransformGroups(this);
+        return this._props.unitSymmetryGroups;
     }
 
-    private _carbohydrates?: Carbohydrates = void 0;
     get carbohydrates(): Carbohydrates {
-        if (this._carbohydrates) return this._carbohydrates;
-        this._carbohydrates = computeCarbohydrates(this);
-        return this._carbohydrates;
+        if (this._props.carbohydrates) return this._props.carbohydrates;
+        this._props.carbohydrates = computeCarbohydrates(this);
+        return this._props.carbohydrates;
     }
 
     constructor(units: ArrayLike<Unit>) {
@@ -110,7 +117,7 @@ class Structure {
         if (!isSorted) sort(units, 0, units.length, cmpUnits, arraySwap)
         this.unitMap = map;
         this.units = units as ReadonlyArray<Unit>;
-        this.elementCount = elementCount;
+        this._props.elementCount = elementCount;
     }
 }
 
diff --git a/src/mol-model/structure/structure/unit/rings.ts b/src/mol-model/structure/structure/unit/rings.ts
index ed22f5418a9944c69b76099e3070d406ec6d4a9d..4b92e5b914f77b682a6c3b5365ae6de87c8affba 100644
--- a/src/mol-model/structure/structure/unit/rings.ts
+++ b/src/mol-model/structure/structure/unit/rings.ts
@@ -6,11 +6,12 @@
 
 import computeRings from './rings/compute'
 import Unit from '../unit';
+import StructureElement from '../element';
 
 interface UnitRings {
     /** Each ring is specified as an array of indices in Unit.elements. */
-    readonly all: ReadonlyArray<ReadonlyArray<number>>,
-    readonly byFingerprint: Map<string, ReadonlyArray<number>>
+    readonly all: ReadonlyArray<ReadonlyArray<StructureElement.UnitIndex>>,
+    readonly byFingerprint: ReadonlyMap<string, ReadonlyArray<number>>
 }
 
 namespace UnitRings {
diff --git a/src/mol-model/structure/structure/unit/rings/compute.ts b/src/mol-model/structure/structure/unit/rings/compute.ts
index 69ffa0506d0499e7870ff19010edf55d95d45405..0225846f647a147a8911211fec2d0efef065dca6 100644
--- a/src/mol-model/structure/structure/unit/rings/compute.ts
+++ b/src/mol-model/structure/structure/unit/rings/compute.ts
@@ -8,6 +8,7 @@ import Unit from '../../unit';
 import { IntraUnitLinks } from '../links/data';
 import { Segmentation } from 'mol-data/int';
 import { LinkType } from '../../../model/types';
+import { StructureElement } from '../../../structure';
 
 export default function computeRings(unit: Unit.Atomic) {
     const size = largestResidue(unit);
@@ -40,7 +41,7 @@ interface State {
 
     currentColor: number,
 
-    rings: number[][],
+    rings: StructureElement.UnitIndex[][],
     bonds: IntraUnitLinks,
     unit: Unit.Atomic
 }
@@ -144,7 +145,7 @@ function addRing(state: State, a: number, b: number) {
     for (let t = 0; t < leftOffset; t++) ring[ringOffset++] = state.startVertex + left[t];
     for (let t = rightOffset - 1; t >= 0; t--) ring[ringOffset++] = state.startVertex + right[t];
 
-    state.rings.push(ring as any as number[]);
+    state.rings.push(ring as any as StructureElement.UnitIndex[]);
 }
 
 function findRings(state: State, from: number) {
diff --git a/src/servers/model/query/atoms.ts b/src/servers/model/query/atoms.ts
index 524338e9fa956e5ae25c046b6f44206bb28151fc..1e0bd7c31f0852c98bcd23788a49d556cf520f36 100644
--- a/src/servers/model/query/atoms.ts
+++ b/src/servers/model/query/atoms.ts
@@ -4,7 +4,7 @@
  * @author David Sehnal <david.sehnal@gmail.com>
  */
 
-import { QueryPredicate, StructureElement, StructureProperties as Props, Queries } from 'mol-model/structure';
+import { QueryPredicate, StructureElement, StructureProperties as Props } from 'mol-model/structure';
 import { AtomsQueryParams } from 'mol-model/structure/query/queries/generators';
 
 export function getAtomsTests(params: any): Partial<AtomsQueryParams>[] {